I noticed this item in a local dollar store and just had to buy a "Wack-A-Pack"
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For a buck you get 4 little packets. Each pack is a "Surprise Greeting" This one is a Valentine message. First you whack the pack. Smacking the pack starts a chemical reaction inside of a tiny mylar balloon that fizzes and inflates the balloon (Vitamin C/sodium bicarbonate/water) As the little balloon inflates, it pops the pack, with a bit of a noise, in a fairly impressive display. For a buck, my son and I had about $10 worth of fun with them