Me: 1 is something, 0 is nothing, NULL is the absence of things
JuniorDev: wut
Me: You've got pizza in a box, that's 1. If there's no pizza in the box, that's 0. If there's no pizza and no box, that's NULL.
JuniorDev: OOH so there's no object to reference if I ask for a slice!
Me: *small tear*

Always explain things in terms of pizza. Always.