Invent something and then figure out what it can be used for later. Like for instance a bag filled with water, you try to lift it up and the sides push away -they resist. Create a container that when you try to lift it it doesn’t resist like that outwardly. Only downwardly perhaps. If you can solve the issue of the latter too lol

Compressible storage containers (accordion) are probably nothing new at this point but the idea came to me some 20 years ago.