I have two rings. I move by request. If the mixture is not right, I just sit at rest. What am I?

The Answer

A teenage boy and his grandfather go fishing one day. While fishing, the old man starts talking about how times have changed. The young man picks up on this and starts talking about the various problems and diseases going around. Teen says, "Grandpa, they didn't have a whole lot of problems with all these diseases when you were young did they?" Grandpa replies, "Nope." Teen says, "Well, what did you guys use for safe sex?" Grandpa replies, "A wedding ring."