Not sure how many people know about this or if this has been posted before but Google loves to add Easter eggs to its services and software for users to discover. These are hidden features or mini-games that exist just for fun.
Here's where to find it (based on Marshmallow):
Step 1: Open the Settings menu and tap on About phone/tablet.
Step 2: Scroll down to Android version and keep tapping on it until you see a large "M" graphic load.
Step 3: Tap on the "M" logo to turn it into a Marshmallow.
Step 4: Press and hold on the Marshmallow until the game screen loads.
Step 5: Tap the "Play" button in the center of the screen to start the game.