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Intro

When she was younger, your little sister used to be a pain to babysit. Whenever she wasn't crying or screaming, she was spitting up. Plus, her diaper was nothing short of a war zone. Things got more interesting (and way more dangerous) when she learned how to escape from her crib and tried fitting silverware into electrical outlets. When she started talking, you got solid proof that the word "poopyhead" could be used over 127 times in under ten minutes. And yes, you counted.

Now that she's about ready to enter kindergarten, she's a lot easier to handle. She calls you by your real name, poops in the toilet, and only cries when boo-boos are involved. You could almost say that you enjoy babysitting her now. Almost. (You keep it on the down low, though. Hanging out with your little sister is hardly an invitation to sit with the cool kids at lunch.)

One evening, you two decide to play her new matching memory game, but it seems to be going nowhere. Every time you flip over two cards that you think match, your sister objects and says it doesn't count. Why? She tells you it's not just a matching game. It's a congruence game. It's not enough to flip over any old pair of triangles because they might be very, very different. For you to win, the two triangles need to be exactly the same shape. They need to be congruent.

When did your sister become so strict about games? Or start paying attention to the rules? Or learn the word "congruence," for that matter? Though, to be fair, she does have a point. It pays to learn what makes two shapes congruent. Maybe not a six-figure salary, but we can promise you that it will come in handy.

For instance, if you are a construction worker building a roof or an architect designing a building or a student taking a geometry class, you'll need to know how to recognize and create congruent shapes. Triangles are among the simplest shapes in the two-dimensional world, so why not start there?

And if it means beating your sister at that silly memory game, even better.

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