\input zb-basic \input zb-matheduc \iteman{ZMATH 2016d.00689} \itemau{Eberle, R. Scott} \itemti{I don't really know how I did that!''.} \itemso{Teach. Child. Math. 21, No. 7, 402-411 (2015).} \itemab Summary: A tessellation'' is a pattern of geometric shapes that fulfills three conditions: (1) No gaps exist between the shapes; (2) The shapes do not overlap; and (3) The pattern can go on forever in all directions. Tilings (such as tiling a floor) are familiar to children and provide a rich resource at all grade levels for learning many different geometric concepts, such as angles and symmetry. This article examines how to organize open-ended tessellation activities in a way that supports the mathematical practices and concepts that teachers want students to learn for geometry. It also looks at the frequently overlooked importance of paying attention to what children find aesthetic in such activities. (ERIC) \itemrv{~} \itemcc{G90 M80} \itemut{tiling; geometry; open-ended problems; tesselations; mathematics and art} \itemli{http://www.nctm.org/Publications/teaching-children-mathematics/2015/Vol21/Issue7/%E2%80%9CI-Don\_t-Really-Know-How-I-Did-That!%E2%80%9D/} \end