Simplify the following text for a Grade 4 native English-speaking person.
Original: By convention , the number 1 is not itself prime , but since it is the product of no numbers , it is often convenient to include it in the theorem by the empty product rule .
Simplified: Further , it can not be a prime number , because a prime number is a ` product ' of a single prime : itself . So it must be a product of numbers .