I stopped by a freshmen math class just to peek in and see the students estimating via the CrockettEstimation blog, but I decided to stay when I saw that they were going to do a Mathalicious lesson.
I wound up working with the same group of four students for about 30 minutes. The time flew by as we discussed their answers and the reasoning behind them. When you have the chance to ask students "Why?" and have them explain their answers to each other is when you truly learn the most from them.