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 group of students is taking a hike along a stream channel. One student picks up a rock from the stream and notices that the rock is very smooth and round. Which type of weathering most likely shaped the rock?
abrasion
Water.