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Block-based programming languages effectively address syntactic difficulties allowing users to more easily create code. Syntactic code support is no doubt a crucial step in enabling the next generation of programmers. However, in what other ways do these tools support the computational thinking (CT) process? For example, how is CT supported in both the semantic and pragmatic levels? Most users do not just want to write code–they want to create interesting artifacts such as video games, simulations, stories and robots. No doubt that users must code to build these artifacts, but just as important, though often overlooked, is that users must also have an understanding of tool affordances to effectively build these artifacts. The position presented in this paper is that the comprehension of affordances is essential to each stage of the computational thinking process. To that end, we introduce the Hourglass Challenge as a way for students to experience affordances through a programming challenge based on mis-aligned tools and problems. The results from a study, with 54 preservice elementary school teachers, indicate that the affordances of tools can have profound consequences on computational thinking. For instance, the affordances various tools provide can be the difference between creating a simulation versus an animation. But just as hammers are not intrinsically better tools than screwdrivers, the contribution of this position paper is not to compare tools, but to promote efforts that enable learners to develop a better understanding of computational thinking tool affordances as essential mediators between problems and tools.