Have you ever needed a D battery in a pinch, but could only find Cs? Since AAA, AA, C, and D batteries are all 1.5 volt cells, they are electronically interchangeable. They differ only in physical size and energy storage capacity. So, if you’re in a pinch, you can cobble together an adapter with a few quarters or other suitable conductive material.
Even a 9-volt battery is made up of 6 1.5 volt cells, which are a bit smaller than a AAA (they are actually called AAAAs!). This could come in handy if all you can find is a 9 volt and you really need a AAA for your TV remote… You’ll just have to spend some time cracking open the 9 volt case with a pair of pliers to get at the little cells inside.