If you are like me you always have a problem deciding what to do with your helmet when you go somewhere on your bicycle. None of my clothes have pockets big enough. In the cinema you can hold it on your lap, but then where do you put the popcorn?
(In Boston's Symphony Hall some of the seats have, or at least had, wire brackets designed to hold a man's hat, brim up, below the seat. Whether it would fit a helmet I do not know.)
A solution may be at hand: Dahon, a maker of folding bicycles, has developed the folding helmet. This video explains:
If you can't see the video, watch it on YouTube here.
So now you can protect your brain-pan and save the planet, and just stuff the folded lid in the pocket of your tux or in your evening bag when you get there.