ELI5: Explain Like I'm 5


Mass is a measure of how much matter there is in an object. Imagine if you could put all of the pieces of something - like a chocolate bar - into a container. Mass is like the size of the container, telling us how much it can fit. Even though the size of something can change, the mass will stay the same. So, if you take a bite out of your chocolate bar, the mass will stay the same even though the size is smaller.