Because if your sandwich basket is missing a sandwich it means that it is not sufficient for its intended purpose which is a picnic, so if you say a person is one sandwich short of a picnic basket then it's a "kindish" way of saying their brain is missing something vital for sufficient operation! it's meant to be humorous and it's better than saying " that person is missing brain cells!"
Because if your sandwich basket is missing a sandwich it means that it is not sufficient for its intended purpose which is a picnic, so if you say a person is one sandwich short of a picnic basket then it's a "kindish" way of saying their brain is missing something vital for sufficient operation! it's meant to be humorous and it's better than saying " that person is missing brain cells!"
The lights are on, but no one's home.
The elevator doesn't go all the way to the top floor.
She's a few clowns short of a full circus.
She's a couple sandwiches short of a picnic lunch
The lights are on, but no one's home.
The elevator doesn't go all the way to the top floor.
She's a few clowns short of a full circus.
She's a couple sandwiches short of a picnic lunch
Also 'He's not the sharpest tool in the toolbox'. (But I sympathize with BC's original doubt; the 'N <things> short of an X' format isn't the most self-evident of idioms.)