Exhaust means to use something up completely or to make someone extremely tired.

When a hiker exhausts her water supply on a long trail, she's drunk it all and has nothing left. When you exhaust every option trying to solve a difficult puzzle, you've tried everything you can think of. Scientists might exhaust all possible explanations for an observation before accepting a new theory.

The word also describes extreme tiredness. After running three miles in PE class, you might feel exhausted, so tired that you can barely move. Parents of newborn babies often complain about feeling exhausted from waking up several times each night. When something is exhausting, it drains your energy: a difficult piano recital or a long day at an amusement park can both be exhausting in different ways.

As a noun, exhaust refers to the waste gases that come out of an engine. Car exhaust flows through the tailpipe, while the exhaust from a jet engine helps push the plane forward. This exhaust is what's left after an engine burns fuel, and it's expelled because the engine has extracted all the useful energy from it.