I don't know what to say - You say this when someone says something so stupid it makes

You say this when someone says something so stupid it makes you want to cry, and you’re just too tired to explain why you’re crying.

'He told me he can run faster than a cheetah. I don’t know what to say.'

'She said she can read minds. I don’t know what to say.'

'He just told me he can live without sleep. I don’t know what to say.'