We all know that February 29th only exists every four years, but do you know what exactly a year is? Or how we came to that measurement? Minute Phsyics does, and they're happy to enlighten you in their latest video.
The scientific explanation of a year is how long "it takes for one physical body to orbit another", so what we define as an Earth year as we orbit the sun is different to what a moon year would be as it orbits us. But enough spoilers, go watch the video, they always explain things far more entertainingly than I can. [YouTube via Laughing Squid]