Why do some balloons float and others don't?

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You may have noticed that the balloons you blow up with your mouth never float up in the air like ones you might get at the store or see at a party!
The reason this happens is that balloons you blow up yourself are filled with the air from your lungs, while balloons that float are filled up using something else: a special gas called helium that usually comes from a special helium tank. 
Believe it or not, helium weighs even less than air! Because helium is so light, it basically “floats on top” of any of the heavier air around it! Even when helium escapes from a balloon (like if the balloon pops) it floats up above the air around it. (We just can’t see it doing so because it looks the same as air!) – This is what makes certain kinds of balloons float up, up, and away!
by   (whyzz writer)
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