One of the issues I found interesting to contemplate when I was a teenager is where do you draw the line in replacing human body parts with artificial parts?

Assume that scientists discover a way of using stem cells to re-create any portion of our body that is damaged. For example, if one of your teeth was damaged, it could be pulled out, and stem cells could be used to grow a new tooth exactly like what you already had, using your own DNA.

If your foot was destroyed in an automobile accident, stem cells could be used to regrow the foot exactly as it was, using your own DNA. You would not be able to tell the difference between the original and the replacement, except that the replacement would not have any scars or other damage.

Assume this technology existed right now. Would you use it to replace damaged parts on your body?

Imagine that you get into a car accident and you need to replace your right foot. Would you be willing to use stem cells to replace that foot? I think everybody would do it.

Then imagine an earthquake occurs in your area, and you lose your left leg. Would you go without that leg? Or would you use stem cells to replace the missing leg?

Next imagine that you develop cancer in your stomach. Would you authorize the doctors to use stem cells to grow a new digestive system for you? Or would you rather die?

Next imagine that your heart fails. Would you use stem cells to grow a new one?

By replacing your parts with new parts, you end up living longer, but as you live longer, you experience more cancer, and more accidents. The longer you live, the more of your parts you have to replace.

Assume that you splatter something flammable in your face, and burn your face, including your eyes. Would you use stem cells to regrow your eyes and face? You probably would.

If your pituitary gland was destroyed from cancer, would you use stem cells to regrow it? You probably would.

Imagine this continues for decades. Imagine that after 150 years you have replaced every component on your body except for your brain. This brings me to three issues that I liked to think about when I was younger.

Imagine that your brain suffered some tremendous damage from cancer or an accident. Would you use stem cells to regrow your brain? If you did, it would be the same brain, but it would not have any of your memories. You would become an adult that didn't know anything. You would have to relearn your language, how to walk, and even how to see with your eyes. Would you do it? Or would that be idiotic? Would that be creating a new and different person? Which component in your body can be replaced without altering you, and which component, when it dies, is the death of you?

Most of us would choose our brain as being the component of our body that we regard as "ourself". We would regard our body as replaceable. Therefore, why not use stem cells to regrow your entire body rather than replace component by component?

In other words, why not behave like airline mechanics who replace parts after a certain amount of time rather than wait for them to fail? Instead of letting your digestive system, muscles, and heart slowly deteriorate through age, why not follow a maintenance schedule in which every 20 or so years you use stem cells to regrow a new, healthy adult body?

Visualize this happening. Visualize the doctors pulling your brain out of your old body, and putting it into a new, brainless, 20-year-old body that has just been grown using stem cells and your own DNA. Would you do that?

If you would replace your components one by one, and if you are willing to replace virtually everything in your body, what is the difference between replacing your body bit by bit, and replacing all of it at one time?

As of today, our ability to produce mechanical arms and legs is crude, but the people a million years in the future might be able to develop robotic arms and legs that work better than our arms or legs. Furthermore, they could design them for different purposes, such as designing some arms and legs for strength, others to be lightweight, and others to handle high or low temperatures. This would allow people to switch their artificial arms and legs depending on what they wanted to do.

If you are willing to replace your entire body at one time, why replace it with a living body? Why not put it into a jar so that it can be protected, and then connect it through a wireless system to a robotic body? By doing that, you would be able to choose which robotic body you wanted to control.

For example, if you wanted to fly, you would connect to a drone or airplane. If you wanted to go into the ocean around Antarctica, you would switch to a body that can swim in ice water, and which has lights built into the shoulders so that you don't have to carry lights. If you wanted to explore Mars, you would switch to a robotic body that can wander around on Mars without any concern about radiation or temperature. You could even walk into a volcano to get a closer look, and it wouldn't matter to you that the robotic body eventually melts.

The space between the Earth and the moon is not truly empty. There are molecules of hydrogen, helium, and other gases flying around out there. What are the chances that the pressure of these gases is exactly the same everywhere throughout the universe? If the gas pressure in interstellar space is different at different locations, will there be a slight "wind" or "breeze" in outer space?

When a star explodes, will it increase the gas pressure in that area, causing an interstellar wind to expand outward in a spherical manner?

On the surface of the Earth, if we could create a perfect vacuum in a chamber, and then suddenly break the walls of the chamber, the vacuum would fill with gases extremely quickly.

The space between the planets is not truly empty. There are molecules of hydrogen and other gases flying around. What is the composition and "air pressure" in space?

Imagine that we have the technology to take every asteroid in the asteroid belt, and send them towards a common area where they all meet at about the same moment in time. Assume that there are just enough asteroids to create a planet with the mass and size of the Earth. In such a case, we would create an Earth-sized planet without an atmosphere. How long would it take for that planet to acquire an atmosphere? And what would the final atmospheric composition be?

If Venus, Earth, and the other planets were initially hot molten balls of rock without an atmosphere, and if they acquired their atmosphere from the gases in outer space, how long would it take Venus to attract as much carbon dioxide as it has? How massive would Jupiter have to be to attract as much hydrogen and helium as it has? After another 4 billion years of collecting gases, could Jupiter become large enough to ignite into a star?

The planets attract meteors and dust particles, increasing their mass, which increases their gravity, which in turn increases the amount of gases they can attract. How much dust and meteors is the Earth accumulating today?

If the dust was accumulating at that rate throughout history, what was the mass of the Earth 4 billion years ago?

If the dust continues to accumulate at this rate, what will be the mass of the Earth after another 4 billion years?

Or, is the Earth losing mass at the same time? Are we losing any gases or dust particles faster than we are attracting them?

The moon, Earth, and other planetary bodies have been hit by lots of meteors, and this implies that there must have been lots of meteors that came close but did not crash into the Earth. Some of them would have passed through the upper atmosphere without getting destroyed.
If a comet or meteor was passing through our solar system, and if it traveled through the upper part of the Earth's atmosphere, could it pick up some dust particles that have bacteria, yeasts, viruses, and other materials?

If so, then that meteor would continue to travel through the universe with living creatures. The creatures would actually be dead, but what would be the effect of those creatures crashing into another planet that had appropriate conditions for life?

Nobody has yet been able to figure out how a bacteria or yeast could appear from a bunch of simplistic chemicals. Perhaps the first organisms were some type of virus-like creatures. In other words, the first living creatures may not have been what we would describe as "living".

If we could have seen those first creatures, we might describe them as "building blocks of a virus". Through time, they may have expanded to the point at which they became some type of crude virus, and eventually some type of living creature.

Scuba divers are warned that when they go to certain depths, nitrogen will dissolve into their blood and body. If they then rise to the surface too quickly, some of that nitrogen will form tiny bubbles, which creates health problems.

What about the other gases that dissolve in our blood and body, such as oxygen and carbon dioxide? What happens to those dissolved gases when a scuba diver rises quickly to the surface? What would the conditions have to be for those gases to form bubbles?

As we ride a train or automobile up and down hills, and as we ride up and down elevators, the gases within our body will be dissolving and releasing as the pressure rises and falls. At what point does this phenomenon cause health problems? For example:

• If an airplane were to rise upward at a rapid rate and allow the cabin pressure to decrease, how high would it have to rise, and how quickly, in order for bubbles to form?
• If there was a tall building with a high-speed elevator, how fast would the elevator have to travel, and how tall would the building have to be, in order for bubbles to form?

Predators try to remain hidden when they sneak up on their prey. One of the interesting aspects of sharks and dolphins is that they frequently swim with their dorsal fins above the surface, which allows seals, otters, and certain other creatures to see them. Why would they advertise their location and direction to the animals that they are trying to catch? Why don't they stay slightly under the water?

Perhaps they want to be at the surface of the water so that they are above their prey, and they realize they are at the surface when their dorsal fin experiences a different temperature and sensation as a result of entering the atmosphere. Therefore, once they feel that sensation of the atmosphere, they realize they cannot go any higher.