On a ride with a buddy right beside me, I once shifted the front derailleur and the chain fell off the outside of the big chainring. My friend could barely call a warning before I had shifted the chain right back into place.
He was amazed. He’d never seen this trick even though he’s an experienced rider. He is one of many who would have stopped to put the chain back on by hand, wasting time and getting greasy. Some of our readers might not know about it yet either.
The next time your chain goes over the big ring or falls off the small one and onto the frame, keep pedaling. Gently! Shift the front derailleur in the appropriate direction. The chain is very likely to catch the teeth and climb right back on.
Do not force it and jam or bend something. If the chain won’t catch, then you’ll have to stop. Look for a twig or the like to lift the chain so you won’t get too greasy.
Of course, this trick is no substitute for a properly adjusted front derailleur! If your chain is coming off more often than Peter Sagan goes for the sprint, get your bike checked.