Neat lil puzzle Julian likes to pose:

You’re at the airport, rushing toward your gate in hopes of catching your flight. You’re just reaching a moving walkway when you notice your shoe is untied. Assuming it is necessary for you to stop and spend around 20 seconds tying your shoe, which will get you to the gate faster:

a) Stopping before the moving walkway to tie your shoe, then resuming running from the beginning of the walkway, or

b) Stepping on the moving walkway, *then* stopping to tie your shoe, then resuming running from wherever you are on the walkway?

Issa Rice has informed me that the puzzle originated here.