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You may not think about the pull of gravity very often, but this powerful force is always acting

on you. Right now, it's pulling you snugly down into your bed.

Gravity is constantly attracting countless gas molecules, creating a blanket of air around

the earth. This blanket allows you to breathe. It's our atmosphere. Take another deep breath

of that refreshing air now. And as you let it out, develop a sense of floating on water.

This is the closest experience we have to zero gravity here on earth.

There's that sensation of weightlessness, playing perfectly still with your arms and legs stretched

out. You can float on the top of the water. Pay close attention to these sensations.

Little ripples atop the surface of the water cause you to sway gently.

As you breathe in, your body floats a bit higher atop the water.

As you breathe out, your body sinks just an inch or two, allowing those ripples of water

to wash across your belly. This is just buoyancy, not zero gravity like in space,

but it's close enough to use as a training ground for astronauts.

Wearing their spacesuits, astronauts dive into massive swimming pools or hours on end.

They carry out spacewalks to get accustomed to the sensations and difficulties of moving in space.

Handles line every surface. Trainees must use one hand to hold themselves in place

while they perform tasks with the other hand. If they let go, they are tethered to the underwater

...