When I was a boy I used to get the bus back home from primary school. On day after I got off, I noticed the narrow ledge under the automatic exit in the middle of the single-decker, and a vertical handle that passengers could use to steady themselves getting on or off. It struck me that if - once the cantilever door closed - I leaped back on I could cling to the outside of the bus and jump off at the top of the hill thus saving myself two or three minutes walking.
By the time the bus got to the top of the hill it was moving too quickly for me to dismount without taking my life in my hands, so I had to cling on in terror all the way to the next stop - further away from home than where I had started.
I saw something of that little boy in Boris Johnson at yesterday's press conference; initially full of bravado, now to scared to jump off and walk instead.