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.