20 people are waiting at the designated meeting point for a pre-booked tour. Guide: I'm sorry, but we can only take 15 people.
General muttering. Guide: I'm sorry, but the venue only allows us to take 15 people.
Further muttering. Guide: Could all those of you with actual bookings go and stand over there please.
Only 10 people go and stand over there. Guide: Yes, I thought so. The rest of you are just trying your luck. Smug couple: But look, there are plenty of spare places, can we come too? Guide: That's not really how this works. You're supposed to book in advance. Smug couple: But there are plenty of spare places, we should be able to come too. Self-righteous woman: I'll be very cross if I can't go on the tour. Guide: Erm.
5 more people turn up on spec, including three small children. Guide: I should warn you there are an awful lot of steps.
All the excess attendees stare at the guide, silently pleading 'pick me'. Guide: I really can't take all of you. The venue only allows us to take 15 people.
5 people slink off. The rest stand firm and look pleadingly. Self-righteous woman: I'll be very cross if I can't go on the tour. Guide: Oh go on, I'll take everyone except you three. Smug couple: Those three are actually with us. Guide: Sigh, oh go on then.
5 more people turn up, clutching printed-out bookings for the actual tour. Group leader: Sorry we're late. Guide: Oh bugger.
25 people set off on the tour.