"Happiness is a Warm Gun" / is a double entendre alluding to a sexual act
A Sudden Realization Ralph meme.
Sudden Realization Ralph is Ralph Wiggum from The Simpsons, eyes wide as something finally clicks. Captions describe moments of sudden understanding - realizing a joke years later, noticing the obvious hiding in plain sight, working out that the snooze button means less sleep. It's the meme for epiphanies that arrive late.