I spend a lot of time here, and I frequently show these comics in class. I use them to teach something (as in, if you understand why this joke is supposed to be funny, then you will understand the principle of ****) or sometimes just to add humour to the class and lighten the mood (which in itself is encouraging of learning).
If you're stuck, then explain xkcd can sometimes help you out, even if the explainations are a bit tedious at times.
And so, in honour of today, I will leave you with this.
Today is Pluto's 82nd discovery-birthday, in case you were wondering... :)