Take a 2 liter bottle, fill it halfway with drano. Put brown play-do around it and call it a volcano. Then, but in balls of aluminum foil and stand back. Krakatoa.
Well, I don't know something exact, but think of something that really annoys you, and try make something that fixes that. Like how someone invented a peanut butter jar with lids on both ends so you can scrape it out easier.