"Based on what I know, it is only possible for this situation to be true."
You got full marks on your test? You must be so happy!
Jim is never late to work. He must be home sick.
She looks so perfect all the time. It must take her hours to get ready.
Loan you $1000? You must be joking!
A teacher? You must be joking! ;-)
I don't have the patience that those engaged in that esteemed profession have. I'm just grateful that they do!