to be one sandwich short of a picnic

[birdeen's call]

I don't get why it means to be stupid. Could you explain it to me?

[englishpro]

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!"

[birdeen's call]

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!"

Thanks for making me the first person you replied to!

[Barb_D]

There are so many ways to say this.

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 list goes on...

[bhaisahab]

There are so many ways to say this.

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 list goes on...

I particularly like "Two bricks short of a load".

[Heterological]

I'm also fond of "a few French fries short of a Happy Meal."

[BobK]

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.)

b

[englishpro]

Not the sharpest crayon in the box is another!