Concealing a large muddy puddle underneath an unstable paving slab, so that when an unsuspecting pedestrian steps on the slab it all squirts out and covers their trouser leg in soaking wet grime, is clearly one of those golden-oldie physical gags that never stops being hilarious. And doing it to somebody who's already had to trudge half an hour out of their way in the rain can surely only make it funnier. Ha ha ha. Oh, my aching sides.