Random memory

[pne]

In chemistry class one day, we watched a video about the noble gasses.

There was a narrator sitting on a big stone or something; several balloons were taped to the edge of the stone. He'd take off one at a time, explain which noble gas was inside, then tell us something about the properties of that gas. At the end, he'd let go of that balloon.

The most memorably bit was the one he described as a "lead balloon"—the gas in question was heavier than air, so instead of floating up into the sky the way the first ones did, it fell fairly quickly to the ground.