Mr. Schidtler asks his class for "the four natural elements of the universe." They respond with "earth, air, fire and water," which compose the Ancient Classic Element System. As to "elements in the universe," there are 90 that occur "naturally," with hydrogen being the most common. Nowadays, and for a long while, students are more apt to be taught the four states of matter - solid, liquid, gaseous and plasma - which would not have set up the scene's joke.
During the locker jokes, Lisa accidentally bangs Alasdair's head with her locker door. He wasn't hurt, but you can hear her say, "Oh, sorry, Alasdair," and laugh a little before going on with her lines.
When asked at Barth's if she'd consider mountain climbing, Vanessa says "I don't know" without getting slimed.