903
xkcd #2929: Good and Bad Ideas
(imgs.xkcd.com)
Alt text:
While it seemed like a fun prank at the time, I realize my prank fire extinguishers full of leaded gasoline were a mistake.
They work based on UV light. So they will not transition inside cars. They also will not transition in cold weather, so you'll walk in snow with clear glasses and have dark ones for 5 minutes once you go inside.
Never had them not work in cold. They change even during heavy snowfall when it's well below freezing and well, no sun, heavy clouds. It's great because they help with the glare from snow.