Few sandwiches short of a picnic?

It's a humorous idiom that compares someone's mental state to being so lacking in common sense that they'd pack a picnic but forget the sandwiches, an essential element.

Here's how you can use it:

* "My uncle is a few sandwiches short of a picnic. He thinks he can fly!"

* "Don't try to explain logic to her. She's a few sandwiches short of a picnic."

It's important to note that this idiom is often used in a lighthearted and affectionate way. However, it can also be considered offensive if used in a mean-spirited or disrespectful manner.