If you need C batteries, a little cardboard or some thick paper can convert some AA batteries in no time.
Sometimes the tools or toys you have require larger batteries than the common AA battery you probably have sitting in a drawer somewhere. YouTuber kipkay recommends a simple conversion if your flashlight needs some juice. AA batteries are the same height and voltage as C batteries, so the only thing keeping them from playing nice is their size.
Cut some thin cardboard or thick paper strips and wrap them around the AAs to increase their circumference and you're all set. Of course, the disadvantage here is that the AA batteries won't last nearly as long as the C batteries. Still, it's better than nothing, and sometimes you just have to work with what you've got.
Turn a AA Battery into a C Battery! [YouTube]