- Rocks interacting with moving water over long periods of time causes weathering
- Sometimes weathering smooths rocks
- So, moving water interacting with rocks would smooth the rocks

- Moving water interacting with rocks would smooth the rocks
- Waves are made of moving water
- Therefore, waves interacting with rocks can smooth rocks

- Waves interacting with rocks can smooth rocks
- A pebble is a kind of small rock
- Therefore, waves interacting with pebbles can smooth pebbles

Therefore, waves interacting with pebbles can smooth pebbles
A student finds a round, smooth pebble on a beach. Which action made the pebble smooth?
- A pebble is a kind of small rock 
- Rocks interacting with moving water over long periods of time causes weathering 
- Waves are made of moving water 
- Sometimes weathering smooths rocks