To the Pythagorean mystics, 10 was the Sacred Tetractys, the number of the Universe, “the source and root of eternal nature.” 10 is the fourth triangular number, the sum of the first 4 positive integers:
The Carnival of Math: Tenth Edition is up at MathNotations, with most of the posts at the middle-to-secondary school level this time. There are too many interesting topics for me to play favorites. Be sure to check it out!
Denise Gaskins' Let's Play Math 