Behold the Prime Factorization sweater, knit by Sondra Eklund. It depicts the prime factorization of every number from 1 to 100.
Okay, look at the bottom row. It looks like there is a blank space on the left. That represents 1, because 1 is the background color, because 1 is a factor of every number. Next is a blue square, which represents 2. Next is a red square, for 3. Then comes 4. 4 = 2 x 2. So 4 is represented by two blue rectangles. Then comes 5. 5 is prime, so 5 gets a new color, yellow. Next is 6. 6 = 2 x 3. So 6 is represented by a blue rectangle and a red rectangle. 7 gets a new color, purple.
And so on. Click through for more pictures and explanation.