Teaching is the best for stuff like this. I teach music and the best answer I've ever had for 'Is the note lower or higher?' was 'I need the toilet, but I can hold it.' I held it together until my mum (who teaches with me) tried to persuade said child that he should probably go now.
Boy: 'But it's a poo and I'll miss the whole lesson!'
Mum: 'X, the lesson's half an hour, it won't take that long,'
Boy (darkly): 'It does, sometimes it does take that long.'
At this point I was trying desperately to continue teaching and managed to hold in the giggles til mum led him out to the loo.