When someone says something that you're not sure about, you can 'take it with a grain of salt.' 

It means you should be a little skeptical and not believe it completely. 

Example:

A: I promise, natto is very tasty!

B: Hmm… I’ll take it with a grain of salt.

It's like adding a little bit of doubt, just as you would add a little bit of salt to your food. 

So, if you hear something that seems uncertain, remember to take it with a grain of salt!