When Steinbeck first begins to describe Lennie, he compares Lennie's actions to those of animals. He walks "dragging his feet a little, the way a bear drags his paws." When Lennie gets to a pond and begins to drink, he dranks "with long gulps, snorting into the water like a horse." These descriptions are the first indication that Lennie is more like an animal than a man.
When George reprimands Lennie for drinking too much water, the dynamic between the two men becomes clear: George tells Lennie what to do because Lennie can't control himself. Lennie was sick last night from drinking too much water and he needs George to remind him of that.
However, Lennie's limited mental capacity becomes obvious when he expresses pride over his ability to make the water in the pond ripple. Once again compared to a bear, Lennie looks to George with excitement when he causes the water to move. This isn't something one adult man would typically seek another's approval over.
"Lennie dabbed 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. 'Look George. Look what I done.'" (page 2)