Okay, I am posting this now because Audrey(check out her comment below) guessed the answer. The correct answer is:
In the first one," a" is repeated. (An apple fell from a a tree.)
In the second one , "the" is repeated. (Molly went to the the store.)
In the third one, "to" is repeated. (Jack went to to the aquarium.)
The reason why so many people don't notice this is because the brain skips over "little words" because we can usually understand it without them. Isn't that interesting? More brainteasers from now on. I love these things.