If a man pre-ejaculates in a lake while wearing pants and 10 days after your period you get rain in your mouth can you get pregnant?

No, for several reasons.

first, pre-ejaculate contains no sperm, and without male gametes pregnancy is impossible.

second, the pants would contain most of the ejaculate, whether in the lake or not.

third, a lake would almost certainly disperse any ejaculate that managed to get past the pants to the point it would have very little chance of finding a suitable female

fourth, sperm would not evaporate with the water and as a result would not be in the rain.

fifth, even if somehow the non-existant pre-ejaculate sperm were to defy the laws of physics and end up in the rain, that you are swallowing for some reason, sperm entering your body through your mouth will never reach your womb, and as a result cannot make you pregnant.

thank you for that delightful romp through impossible scenario land. now then, on to a bit of self-assessment, if you posted this as a joke, you are a strange person and I cannot possibly conceive of the reason. If you are serious, and honestly wanted to know if this completely impossible scenario could happen, please see a psychologist, you need to develop a better sense of reality.

thank you for your time.