Just when you thought you'd heard it all when it comes what the cast of Harry Potter got up to on set, Alan Rickman comes along and delivers a whole new nugget of joy.

During a visit to see Jimmy Fallon on The Tonight Show last night (June 17), Professor Snape dished on one of his most childish pranks yet as he teamed up with Michael Gambon (Dumbledore) during filming.

The actor reveals he and Michael did indeed hide a fart machine in Daniel Radcliffe's sleeping bag, in a scene with 200 other children no less.

But as is always the case, the story is so much better when told in Alan's silky smooth voice.