MYTHBUSTERS
'(<a href="http://www.j-archive.com/media/2011-10-27_J_18.wmv">The Mythbusters deliver the clue.</a>)  The idiom "going down like a lead balloon" implies that lead balloons can't fly but by constructing a balloon & filling it with enough of this inert gas, we achieved lift-off, myth busted!'
helium