How do you add words?

Seems the older boys are learning how to ‘add’ words. Making compound words. Now they have already done this but it was something they started for some unknown reason to me. And they kept laughing about it. And once even I laughed. My youngest had to join into it. Now he loves to be funny and will think about ways to be funny. This time took the cake. He asked the older boys what “Poo plus Pee equals”. We all gave him blank looks. He said “Poopy”. Ok, we all laughed so hard at that one. It was funny. Hard to believe in 4 short years instead of poopy jokes, it will probably be acne jokes and I’m sure it will be while I’m out look for acne medicine. I actually think I might look forward to it. It might just be funny!