Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Two pool (billiard) balls collide, causing each to move in a different direction. Eventually, both come to a stop. The most important factor in causing them to stop is
friction.
Friction.