The problem with birdie air locks

We went to the zoo this weekend and enjoyed all the new habitats that they've been working on constructing for the last year or so.  They're largely big aviaries, incorporating all the native bush and netted-in over their tops.  The entrances are cleverly constructed to prevent the birds getting out: you have to go through two consecutive sets of doors, only one of which can be opened at a time.  The brothers loved taking the responsibility of opening the doors for us.

You may be able to anticipate the problem that developed.  Arram got a little ahead of the group, eager for door duty.  First he opened one aviary door, went through, and closed it behind him.  Then he trundled over to open the second door.  Attempting to go through, however, he got a little off-balance as two-year-olds do, and tumbled bottom-first into an inconveniently-placed mud puddle.  This was unacceptable.  He wailed.  The door eased toward the jamb but fetched up against his little bottom, and was thereby propped open.  Oh dear.  This meant that Mama, on the other side of the first door, was locked out and could not do the rescuing that was so desperately needed.  It was an earnest minute with all of Arram's zoo friends urging him to stand up and allow the door to shut.  Eventually he did and big cuddles followed.  He even got a cookie.  His bottom wasn't even very wet; it was a small puddle.