First appearance of Mr. Treeger.
When Phoebe says that she feels that she is not pulled but pushed by gravity, she is actually right. Albert Einstein's General Theory of Relativity states that what a person feels as gravity is actually the curvature of space-time due to the mass of the earth and this curvature is pushing the body toward the center of the earth.
Joey asks if he can have the big magnifying glass in Heckles' apartment. This is the second time he asks for it. The first time was in The One Where the Monkey Gets Away (1995).
When Ross is arguing with Phoebe about evolution, he says it's "scientific fact". While "theory" may mean a hunch or a guess in nonscientific terms, "theory" means something that can and has been proven to be fact, e.g. the theory of gravity.
Mr. Heckles lives in apartment No. 8. Monica and Rachel live above Mr. Heckles in apartment No. 20 with Chandler and Joey living directly across the hall in apartment No. 19.