The same material that makes them bounce gives off a horrible stench, one that is rubbery + plastic-ky at the same time. It's so horrible that even if one of those balls were removed from the plastic packaging they come in, the nausea/migraine inducing stench will engulf the entire room.
I've placed the two balls in a sealed container and will get rid of them on Monday when the garbage man arrives. What a waste of 5 dollars and 40 cents.
But if you want to get them as stink bombs, be my guest. They are available at USD 2.90 each at Play-Asia.com. Click here for purchase details.
DON'T SAY I DIDN'T WARN YOU!