Fibula, the bus timetable reminds me of one of my favourite teacher jokes:

A teacher in a rural school is trying to use an example to teach subtraction. She says, "If I have nine sheep in a paddock and one jumps the fence, how many sheep do I have left?"

Little Johnny says, "None".

"No, that's incorrect. There were nine sheep, one jumps the fence, how many do I have left?"

Johnny says, "Miss, you have no sheep left".

"No, Johnny" says the teacher, "you don't seem to understand. I have nine sheep, one jumps the fence, how many have I got left?"

"No, Miss", says Little Johnny, "YOU don't understand. If one sheep jumps out, they all follow."