Some Hollywood producers think box office success is as easy as a mathematic equation: Hot celebrity + hot celebrity = sizzling on-screen love story and tonnes of money.
But we all know that's not how romance works in real life and it's not how it works in the movies, either.
What is chemistry, anyway? It's a spark or a connection or a gut feeling that two people really belong together. Or that they should at least hook up.
It's a tough thing to define, but we know chemistry when we see it on the big screen and we know when we don't see it, too.
Here are our picks for the top 10 least believable movie couples ever...