Given the task definition and input, reply with output. In this task, you will use your knowledge about language (and common sense) to determine what element the marked number refers to. The numbers are marked with two underlines around them, like: _ number _. Your answer should be chosen from the given text, and should not contain other words.

Baby Sinclair: What 's Refrigerator Day ?
Ethyl: I already told you .
Baby Sinclair: Oh , was I listening ?
Ethyl: Refrigerator Day is the day when dinosaurs celebrate the one invention that made modern civilization possible .
Baby Sinclair: Diapers ?
Ethyl: No , although that was an important _ one _ too , especially in summer .
invention