I can easily remember waaaay back in public school when part of the class would finish an assignment ahead of the rest of the class. We were, in such cases, usually "rewarded" with more work. But, sometimes this came in the form of riddles or puzzles. These I enjoyed. They didn't feel so much like work. Yet, little did I know that these exercises were helping to train my mind for critical thinking, spacial reasoning, and other thinking exercises.
Tangrams are just such an exercise. I'm getting ready to introduce this puzzle to my kids so they have something to work on during the short pauses of learning that happen in a day - you know, something other than bothering a sibling or asking for a snack. This Tangram site is very helpful for this purpose. Along with instructions on how to make your own tangram pieces, it contains lots of puzzle shapes to work from. See if you can figure out how to arrange the pieces just right so they look like the given silhouette.