On the other hand, sarcasm can be very negative.
That's why early on in the year I try to really teach my students the difference between the two.
Funny things happen all the time in our class. We stop to laugh with someone, laugh about a story, or laugh when we tell a joke.
When you're sarcastic, it sometimes cuts like a knife. If you let it go on (or use it yourself), it can be very hurtful to others, and you'll have big problems later on.
That's why, at about Day #10 (which was today)... I start to get a little "goofy" and tell some jokes, and smile a lot, and generally just show the students that I enjoy life.
I also use it as an opportunity to let them know where "that line" is.
As the students look on the floor, I explain to them that it's OK to have fun, but not at another person's expense (and that there's really not a line on the floor... it's just a figure of speech).
Just be careful to not cross that line yourself.
Remember that you're a role model and they will go as far as you take them.
So, if you like "knock - knock" jokes... go ahead and tell one. See what happens, because you'll probably get about a thousand "knock - knock" jokes right back at you.
The humor that I like to portray is one of using different voices, or pretending to act out a funny scenario. This allows the students to feel comfortable, and then you'll start to see them doing it during some of the "down times" in the day with each other (like at recess, or snack time, or at the end of the day.
It's pretty neat to see how they react to it, and you definitely get to know them much more when they start to laugh, smile, and feel comfortable with their peers and ultimately in your classroom.
Isn't that the goal?