When given the opportunity to play with mathematical materials and ideas, children demonstrate their mathematical understanding in innovative ways. The open-ended nature of the "Pattern Blocks" activity provides agency for children to decide how to approach the tasks, ways to differentiate beyond more common pattern-block puzzle choices, and opportunities to demonstrate a variety of content and practice standards. Although the examples for this article were from a kindergarten classroom, we believe that similar opportunities at any grade level may provide a chance for young mathematicians to employ their imaginations and thereby demonstrate what they know.