A husband suspects his wife is having an affair. He needs to go on a business trip for several days, so he decides to set a trap for her.
He puts a bowl of milk under the bed. From the bed springs, he
suspends a spoon. He has it calibrated so that her weight on the bed
will not drop the spoon into the milk. But, if there's any more weight
than that, the spoon will drop into the milk and he will detect it upon
his return home.
He comes home several days later. The first thing he does is reach under the bed and retrieve the bowl.
The bowl is full of butter.