In this paper, we show results suggesting that Tangible User Interfaces (TUIs) can be used to prepare students for future learning. In a previous study, we found that students who used an interactive tabletop first and then studied a text significantly outperformed participants who read a text first and then used the tabletop. This finding shows that discoverylearning approaches are more suited to TUIs than traditional "tell-and-practice" approaches. In this paper, we generalize this effect to a different population, a different learning material and a different topic. The tangible interface, Combinatorix (Fig. 1) enables small groups of students to collaboratively discover concepts in probability. Our system supports students' exploration of principles in combinatorics (i.e., permutations and combinations) that serve as foundations for learning about probability. We describe the design of Combinatorix, as well as an experiment that examined the interaction between focused lectures and free exploration. We found that students who first explored the topic on a tangible interface and then watched a video lecture significantly outperformed students who watched a lecture first and then completed a hands-on activity. We discuss how the "functional fixedness" induced by the video lecture limited students' learning of probability, and conclude with guidelines for implementing interactive tabletops in classrooms.