This is the only knock knock joke I have ever heard since puberty that I actually find funny - it is a somewhat funnier and more subtle form of the interrupting cow joke. Most people past their teens with an iota of humor seem to enjoy it too. The joke teller starts the joke as if in a hurry, and rushes the responder a bit throughout the joke leading to the punch line...
"Impatient cow who?" (Before the responder finishes speaking this line, the joke teller interrupts with the punch line...)
I don't know why I did this to myself a second time - noding a joke, that is. It didn't go over well the first time. You'd think people would see the reflection of societal issues that exists in jokes and uphold their value on E2, but, I guess not...
Author's note: Do NOT try to tell this in a hospital or health care setting. The word impatient sounds too much like inpatient, and will invariably cause the joke to be spoiled. Also, cardiologists don't seem to be able to get this. I don't really know why.