This app does tesselate! It provides a selection of existing shapes (square, triangle, etc.), and wherever you touch and drag within the corners of those shapes, the app responds by creating a live tessellation (and also shows the repeating pattern in the background). Very nice to see the pattern immediately!
Now, this could become cooler and way more useful (not to mention educational) in the following ways:
(1) allow the end points (say the peak of the triangle) to move. Right now the shape’s points are pretty much fixed in spot, despite all the changes you can make elsewhere. I would call this the “edit” button
(2) an undo/redo option would be nice for quick test runs
(3) create a “curves” button where you press, hold, and drag to introduce a bend in your line. Groovy.