Once the game starts, you can direct a transparent snake around the video player with the arrow keys to gobble up dots that cause the snake to grow. Hitting the edge of the player or any part of your snake's body will end the game.
The Easter egg — just one in a long series of similar stunts from Google — was discovered by users last week. No one's sure exactly when the game was added, but it must be recent because it only works on the new YouTube video player introduced in April. It doesn't work with embeds, either, so you'll have to go to YouTube's website to try it out.
Previously, Google put a button on YouTube videos that played a World Cup-style vuvuzela sound. Google's no stranger to playable games on its websites, either; the logo on the Google home page became a playable game of Pac-Man two months ago, supposedly costing society more than $120 million in productivity.
This video that was embedded at gaming blog Kotaku reveals the feature, but note that the creator's claim that you must be in a "Gaming" category video to play Snake is incorrect. Otherwise, it's a solid explanation.