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: A student sets a block at the top of a wooden ramp. The student pushes the block. As the block slides down the ramp, it slows down and then comes to a stop. What force causes the block to slow down?
the force of friction
Friction.