How do you get a cat and dog to get along?

please! I have a cat,Tigger, and a dog,Molly, that dont get along. Molly seems like shes willing to get along but Tigger always hisses and growls at her when she gets near and everyone's tired of it. Does anyone have any ideas? plz and thank you!

Here's one: Put one of them, maybe Molly (the dog), on the outside of a clear glass door, then have Tigger (the cat) on the other side. Let them study each other for some time (possibly a few minutes or several hours, or until one of them wants to go back inside). Then move it closer: inside. But only when they seem to be okay with each other. Could one of them be pregnant, thus causing jealousy? Or perhaps one of them had a bad experience with the other kind before? In either case you should bring them, either together or at different times, to their vet and have them (the vet) check them out. HTH.