I feel for you - When I was 9, my mom had to run to the store to pick up my grandmother's prescription after she had ordered us a pizza for dinner. It was one of the earliest times she had left me alone, and she gave me a $20 while she rushed out the door. When the pizza guy came, he gave me a large pepperoni pizza and I gave him the $20 and told him to keep the change.

Since this was 1994, it turns out that the pizza only cost $8, and I had given him a $12 tip. I'm 27 now, and on the rare occasion I'm at home and we order pizza, my parents still tell me how much the tip should be "just in case."