The reductio ad absurdum is an argument form which works backward from an absurd conclusion to show that a premise of the argument must be false. For example:

1. Nothing can do what is physically impossible.
2. It is physically impossible for bumblebees to fly.
3. Therefore, bumblebees cannot fly.

However,

4. Bumblebees do in fact fly.
5. Therefore, either premise one or two must be false (in this case, premise 2).