During a slow swim this morning I was running through the first few lessons I'm planning to teach, and one of them is about primes. I was trying to come up with good questions for primes. Is 158 a prime? No need to check cos it's even. Is 9 a prime? No although it's an odd number. Or is Pi prime (cos there's only two numbers that go into it, but it's not an integer). How about -7? And here is where I forgot about swimming. Initially I'd thought that -7 would not be prime, because its factors could be -1 and 7 OR 1 and -7 which means there are two many factors (although someone will argue that they are the same number). So far so good for me. But then I thought about normal 7. That could be 1 and 7 OR -1 and -7. And that's too many factors as well! Have I forgotten something about the definition of primes that you are only allowed to consider positive integers as factors?