Q: Toss two coins. Let X = number of heads. Describe X. X is discrete: values 0, 1, 2. P(X=0) = 1/4 (TT), P(X=1) = 2/4 (HT, TH), P(X=2) = 1/4 (HH). Contrast: let Y = waiting time for a bus (continuous): P(Y = 5 exactly) = 0, but P(4 < Y < 6) > 0!