I can't think of the 'funniest' but this one popped into my mind and I had to re-watch the other day. Also liked their "are we the bad guys?" Nazi one.
Was teaching in Islington and a kid from another class came in and asked for four glues (obviously this story works better verbally). I said, sorry you want some tiny toilets for cutlery? The child looked perplexed as thirty others had a giggle.