How do you connect copper pipe to pex tubing?

The copper should fit in the Pex fitting. My mistake, Pex uses the crimp rings. There is a coupling and other fitting that you just push the pipe into and it grabs and seal the pipe. Pex, pvc, copper, all will fit. Just use a coupling between the copper and the pex. I had my doubts the first time I used this, but it does work and doesn't leak. Just make sure that the pipe bottoms out in the connection.