Quadruple A Batteries
Most of you have heard about the double-A battery or also known as the AA battery. Most of you have also heard of the triple-A battery. But, have you heard of the quadruple battery A.K.A. AAAA battery? No? Me neither.
This battery is also used in other small devices such as laser pointers and glucose meters (according to Wikipedia).
The battery is very small (smaller than the AAA battery) and it last about a year.
Note: It also feels weird to hold it.