Here's a pretty strange idiom from the world of British advertising.
It does (exactly) what it says on the tin is a phrase used to say that something does exactly what it says it does, without fail, in a very straightforward way.
It doesn't have to be limited to something sold in a tin and can even be applied to political campaign promises, as done by UK Prime Minister, David Cameron in his 'Ronseal deal' speech in 2004.
The marketing department of wood protection product company, Ronseal, can be very proud that their advertising slogan has become part of everyday language.
I told you I would bring you idioms - and I have. This site does exactly what it says on the tin.
Here's the ad: