What was the first bit of mathematics that made you realize that math is beautiful? (For children's book) I'm a children's book writer and illustrator, and I want to to create a book for young readers that exposes the beauty of mathematics. I recently read Paul Lockhart's essay "The Mathematician's Lament," and found that I, too, lament the uninspiring quality of my elementary math education.
I want to make a book that discredits the notion that math is merely a series of calculations, and inspires a sense of awe and genuine curiosity in young readers.
However, I myself am mathematically unsophisticated.

What was the first bit of mathematics that made you realize that math is beautiful?

For the purposes of this children's book, accessible answers would be appreciated.

(Copy from http://mathforum.org/library/drmath/view/57919.html)
There is a well known story about Karl Friedrich Gauss when he was in
elementary school.  His teacher got mad at the class and told them to
add the numbers 1 to 100 and give him the answer by the end of the 
class. About 30 seconds later Gauss gave him the answer.  
The other kids were adding the numbers like this:
$$
1 + 2 + 3 + ... + 99 + 100 = ?
$$
But Gauss rearranged the numbers to add them like this:
$$
     (1 + 100) + (2 + 99) + (3 + 98) + ... + (50 + 51) = ?
$$
If you notice every pair of numbers adds up to 101.  There are 50 
pairs of numbers, so the answer is 
$$
50 * 101 = 5050
$$  Of course Gauss 
came up with the answer about 20 times faster than the other kids.
In general to find the sum of all the numbers from 1 to n:
$$
1 + 2 + 3 + 4 + ... + n = (1 + n) * \bigg(\frac{n}2\bigg)
$$
That is "1 plus n quantity times n divided by 2".
