A nursery rhyme for very small children. Both players clap their hands against one another's while chanting:

Patty-cake, patty-cake, baker's man
Bake me a cake as fast as you can
Roll it, and pat it, and mark it with a "B"
Then put it in the oven for baby and me!

... with the appropriate pantomines for rolling, patting and marking.

Obvoiusly, there's no official Patty Cake Rule Book to define the handclaps and the pantomimes -- it probably varies from person to person. (BTW, experienced bakers will tell you that baking a cake "as fast as you can" is a recipe for disaster.)

Update: pimephalis informs me that Patty-Cake is derived from the Mother Goose rhyme "Pat-A-Cake."