*Knock on Wood*

Knocking has different roles in Germany.

-When a professor is done teaching, all the students knock on the desks as appreciation. Yup, that was a huge surprise to me.
-All office doors are closed, even during office hours (seems ridiculous to me), so you have to knock and wait for them to say "come in." (Of course, this is especially complicated if you aren't sure what they've said.) After entering, you close the door behind you. I will never ever get used to the whole office hours thing. They're not there for you, you're there for them, so it doesn't matter what you have on your schedule, if you want them to help you, you'd better show up for the couple of hours they have scheduled. (And yes, this even applies when you're calling from a foreign country. They told me to call back during office hours which were during the middle of my night.)
-Sometimes people will come in to a gathering and instead of greeting everyone individually, will knock on the nearest table or doorjamb and wave at everyone.