Ever since S1 started going to school on the school bus, S2 has been insanely jealous of his bus-going habits. She's wanted to go on a bus for a long time, but in Norman that's kind of hard. There's a metro bus system, but it doesn't go the places we normally go.
In London Heathrow, the terminals are far apart and to get from one terminal to another, you need to take a bus. To adult passengers this is a royal pain, because the bus takes you out of the secure area. You need to go through security again at the next terminal. But to S2, there was nothing more wonderful than to be able to get on a "city bus". And that's all she would talk about to every stranger who cooed at her for the next couple of hours.