Only a six year old boy looks out the van window, sees a squirrel running down a tree truck and asks, "Mommy, how many balloons would it take to lift a squirrel?"
First, we need to know how much a squirrel weighs. I think we have grey squirrels around our house, which Wikipedia says have an average weight between 400 and 600 grams.
Next, we need to know how much a regular helium balloon can lift. How Stuff Works says that a typical amusement style balloon can lift 14 grams, if you ignore the weight of the balloon and the string.
So, for a small squirrel, you would need 400 / 14 = 29 balloons, (plus several balloons to account for the strings). For a large squirrel, you would need 600 / 14 = 43 balloons (plus several more ballooons to account for the strings.)
And now we all know.