I bought a 1940 built house with two prong outlets. I'm replacing a celing fan the previous owner installed. There are only two wires coming from the celing outlet and both are black. The non-contact voltage tester I bought for $10 lights up on both wires. I noticed this voltage tester lights up on both sides of most receptacles in the house.

Do I have a major problem or is this normal? How should I select a wire for the ceiling fan for hot?

Thanks a lot.