Teachers use prime and composite numbers charts to teach students to identify prime and composite numbers as they shade them with two different colors. A prime number is a number greater than one that has only two factors, itself and one. A composite number has more than two factors.
Students are given a square chart of the numbers 1 to 100, or the numbers 1 to 1,000, and told to shade prime numbers in red and composite numbers in blue. This is a simple exercise when dealing with the smaller numbers. For example, eight of the first 20 numbers and 15 of the first 50 numbers are primes, but only 10 of the next 50 numbers are primes, for a total of 25 primes in the first 100 numbers. In the following 100 numbers, there are 21 primes, and in the 100 numbers following that, there are 17 primes.
A clever student, taught to make conclusions from trends when dealing with numbers, may conclude that eventually prime numbers disappear, and that after a certain point, all numbers are composites. This, however, is a misconception. Prime numbers continue to appear infinitely, as proven first by the Greek mathematician Euclid. Even in a prime and composite numbers chart that goes to a 1,000, a student has to carefully distinguish a prime from a composite number.