When my oldest was about 4, he could not keep a secret. His dad took him Christmas shopping and managed to convince him NOT to tell me what they had purchased for me. When they came home, my boy very proudly said to me that he wasn't going to tell me what my present was, but that they had got it at the slippers store.
One day while doing laundry, I asked my youngest son if he was out of pants. He replied "No, I'm in of pants" and it's become part of our family's vernacular.
Other cute things my children have come up with over the years: Instruction sheets for things you put together are "constructions". Apartments are "compartments".