Lennie dabbled his big paw in the water and wiggled his fingers so the water arose in little splashes; rings widened across the pool to the other side and came back again.
Lennie is proud of his ability to create movement within the water. The ripples and small splashes impress him. He tells George to look at what he did after the above narration in the book. This shows the simplicity of his mind since this is such a regular task. It also demonstrates he can be entertained very easily.
On their way to their new ranch at the beginning of Of Mice and Men, Lennie and George stop by a pool on a hot day. Though the surface of the water is green, Lennie "drank with long gulps, snorting into the water like a horse" (page numbers vary by edition). George becomes nervous that Lennie will get sick from drinking from standing, not running, water. Then, Lennie does something of which he's proud:
"Lennie dabbled his big paw in the water and wiggled his fingers so the water arose in little splashes; rings widened across the pool to the other side and came back again. Lennie watched them go" (page numbers vary by edition).
The irony is that Lennie is very proud of making rings in the water and watching them return to him. From this small episode, the reader knows that Lennie is childlike and doesn't appreciate that he is perhaps drinking unsafe water. Instead, he is focused on what a small child might like—making ripples in the water. The reader also appreciates right away that George is Lennie's constant caretaker and must keep watch over him, as Lennie is not always able to exercise sound judgment.