The Cuddly Duddly Fuddly Wuddly Riddle

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You’ve promised to get your son the cutest creature in creation: the cuddly. It’s part of the Wuddly species, cousin to the terrifying duddly and the hideous fuddly. To make one, 100 eggs are placed in an incubator to undergo egg fusion, and the resulting combination will produce either a cuddly, a duddly or a fuddly. Can you make the right combination to get a cuddly? Dan Finkel shows how.


For your son’s sixth birthday, you’ve promised to get him the cutest creature in creation: the cuddly. It’s hard to believe that it’s a cousin of the terrifying duddly or the hideous fuddly. They’re all members of the Wuddly species, and the process of adopting them is deeply peculiar.

It takes 100 eggs to make a single animal in genus Wuddly. When 100 eggs are placed together in an incubator, they undergo egg fusion, and combine in the following way. Blue and purple combine to make red eggs. Red and blue combine to make purple eggs, and red and purple combine to make blue eggs.

The most plentiful eggs pair up first, and if two piles are even, an egg comes from one of them at random. They keep combining until there’s just one left. If the final egg is blue, a Cuddly hatches out of it. Purple eggs give you Duddlies, and Red eggs give you Fuddlies.

The incubator currently has 99 eggs in it. 23 are blue, 33 are purple, and 43 are red. You can begin the process of egg fusion by adding an egg of any color to the room. When all the eggs have combined into a single egg, the creature that hatches will bond with you on sight, which is why getting a Cuddly is so important. After all, you made a promise to your son.

Which color egg should you add to the incubator to get a cuddly?

It’s easy to get mixed up with all the cuddlies, duddlies, and fuddlies coming from different colored eggs. If we ignore how many total eggs of each color there are, and just look at the process of egg fusion, we might notice something that will make this problem simpler.

When two eggs fuse, the number of eggs of each of those colors decreases by one, and the number of the third color increases by one. That means they all change parity, or evenness and oddness, at the same time. Right now all three piles are odd, but you get to add an egg to one color, which means that it’ll be even and the other two will be odd. Whichever color you choose will always be the opposite parity of the other two piles: odd when they’re even and even when they’re odd, since every egg fusion flips each pile’s parity simultaneously. We want to end with 1 blue, 0 purple, and 0 red eggs, or odd, even, even. That means we want the blue egg pile to be the opposite parity of the other two piles at the start as well.

So you add a blue egg into the room, and 99 egg fusions later, only a single blue egg remains. The Cuddly that hatches is sure to make your 6-year-old as happy as can be. Just be sure to follow the shopkeeper’s warning, and never feed it after midnight.

Ali Kaya


Ali Kaya

This is Ali. Bespectacled and mustachioed father, math blogger, and soccer player. I also do consult for global math and science startups.

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