Stu, Chazz & Howard take the Rugrats to the zoo, where the employees wear animal costumes. After noticing this, the Rugrats learn that animals are people, too, as they see these "animals" do human stuff like eat ice cream, play with toys and sell souvenirs. When placed in a caged petting zoo, they think that they will turn into animals and remain there forever and ever. Later, when Tommy dropped his trusty screwdriver outside the pen, the Rugrats call on Dil and the real animals to help him out. (thetvdb.com)