You probably know what a storm is - that's a type of extreme weather with strong winds and rain or snow but what about a storm in a teacup?
A teacup is usually a small, thin, delicate cup for drinking tea - with milk of course, if you are British!
Now back to the storm - a storm in a teacup is something that is something and nothing; it's a small argument that gets blown out of proportion and is not really worth worrying about. Like all storms, it will pass eventually.
This last year as seen some very strong storms in Britain, particularly on the south and west coasts; like this! (and this definitely isn't a storm in a teacup)