Maak dat de kat wijs

If you do not believe what a speaker says, you might use one of the following expressions, building from polite to more informal/strong:

Dat lijkt me sterk

That doesn’t seem to be true

Dat geloof je toch zeker zelf niet?

You don’t believe this yourself, do you?

Maak dat de kat wijs!

Tell that to the Marines (‘You’d better convince the cat’)