Booby trap

We never left the house today so I had to look around indoors for a blip.  I thought this might do.

It's a biscuit tin.  One of W's friends bought it for her birthday last month.  It looks fairly innocent, but here's the thing.  It doubles as a music box; wind the top around clockwise and it rotates back and plays 'Happy Birthday'.  Of course, being clockwork it goes for a while and then runs down so it's not going forever.  But there's clearly a bit of latent energy left in the spring, because if you try to quietly take the lid off and sneak a biscuit out of it, it invariably plays about three notes.  Just enough to warn someone with good hearing that someone is stealing their biscuits.

I've been caught twice so far.