Originally Posted by Cat Herder:
I just checked them all "met". 
I went with the simplest thing I could come up with, anatomy.
Specifically, Pull-Ups. I put two stacks in the bathroom, pink and blue. They went in one at a time and picked their own. Done. 
Surely that won't offend?
Blue are made with more absorbency up front for the location of a urethra in a penis and pink have more in the middle for the location of a urethra in a pelvic floor.
Good? Bad? I mean they are 2-3 years old.
I had one boy come with the pink pull ups. You know, pink is his favorite color and he picked them!