What does "like two peas in a pod" mean — and why is it funny?

informal

Meaning

Two people or things that are extremely similar, or always found together.

Where it comes from

An old comparison, in use since the 1500s. Peas grown in the same pod really are nearly identical.

Why it is funny

The gentle humor is the cosy snugness of it. Two friends become two little peas tucked side by side in one pod — matching and inseparable, which is sweet and faintly silly at once.

Used in a sentence

"The twins do everything together — like two peas in a pod."