Astronauts floating around inside the space shuttle are not actually in a zero-gravity environment. Explain what is the real reason astronauts seem weightless.

For the answer to the question above, imagine they are traveling at approximately 17,500mph, at a tangent to the Earth, so, in absence of gravity, would be propelled away from the Earth like a slingshot. But the Earth's gravity pulls them in at the same rate, so that the net effect is zero, and they circle the Earth instead of flying off at the tangent.