What does "throw a spanner in the works" mean — and why is it funny?

informal, British

Meaning

To do something that suddenly disrupts a plan or stops it from working.

Where it comes from

A British phrase from the early 1900s — Americans say 'throw a wrench in the works'. A spanner dropped into machinery would jam and wreck it.

Why it is funny

The humor is the cartoon vandalism of it. A smooth-running plan is pictured as delicate machinery, and ruining it is as simple, and as satisfyingly destructive, as lobbing a tool into the gears.

Used in a sentence

"The sudden rain threw a spanner in the works for the outdoor wedding."