What's an evaporation line on a pregnancy test?

If you don't throw away a negative test, and you check it again later, there may be a faint line that looks like a positive, but it isn't. That's an evaporation line. You DON'T get evaporation lines during the time frame you are supposed to check the test, usually no more than 10-15 minutes after you pee on it. (Assuming you aren't setting the test down on the concrete outside in North Texas this summer, then you would probably get an evaporation line before you could get a result!) So if you put it down and forget to check it during the correct time frame, it DOESN'T MATTER if there is a line later on or not. It's no longer valid and you have to test again. Whatever happens after the first 10 or 15 minutes doesn't count.