What has Charlie learned about Algernon's motivation to learn?

Charlie has learned that Algernon's motivation comes from food, but he's looking to find other rewards that will induce him to solve problems.

I plan to build him a three-dimensional maze out of scrap plastic that I can pick up cheaply downtown. There are some complex maze variations I'd like him to learn to be sure he, keeps in shape. But I'm going to see if I can find some motivation other than food. There must be other rewards that will induce him to solve problems.