The earth is round and it rotates. If we are standing on the earth, why don't we fall off the earth when "our side" is upside down? The answer, of course, is gravity.

Hi, this question could be interpreted in two ways: Scientific (see: Holly) and Ironical (see: Craig). That's why is very important to provide the context of your question. Best wishes (pa pa)

This question was already answered very well by Holly. I want to add something you might like. "(He, she, you) fell off the face of the earth" = (he, she, you) dissapeared and is no where to be found.

In English, we use the word "face"to talk about the surface of a shape that has 3 dimensions, height, width, and length. The earth is a sphere, which is a ball. So the face of the earth is the surface of the earth. "Why do we not fall off the face of the earth" is asking why do we not/why don't we fall off the face of the earth. I hope this was helpful for you.