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Oh, I'm a Yummy, tummy, Funny, Lucky Gummy Bear
Yummy means that it's delicious, (of food) delicious, and so tummy would mean it's yummy (delicious) in your tummy; a person's stomach or abdomen. And funny means it's causing laughter or amusement; humorous, or difficult to explain, strange perhaps. Lucky means that he's having, bringing, or resulting from good luck, and so it's saying that he's a strange gummy bear in your abdomen and it's so hard to explain, that it brings amusement to you and luck. Or as he says, "Yummy, tummy, Funny, Lucky Gummy bear.