Stochasticity

As part of a recent Radiolab episode on randomness, the hosts visit Statistics Professor Deborah Nolan at the University of California, Berkeley. She has the class do an exercise: while she is out of the room, two groups each generate a list of one hundred coin flips. One group flips a penny and records the results as a series of H's and T's, the other group imagines they are flipping a coin and writes what they suppose the results would be.

The two lists are then written on the chalkboard. The professor returns, takes a look at the two lists, and promptly identifies which list is the result of actual coin flips and which one was faked.

How did she do it?