What is an Insect?

What makes an insect an insect? Is it a very small, many-legged creature that flies? That's a good way to describe some insects, but others, such as ants, never take to the air at all. And just how many legs do insects have? A beetle has six legs and spider has eight--are these both insects? Knowing whether or not an animal is an insect can be tricky, but What is an Insect? makes solving the puzzle fascinating and fun.