How does something become Najis?

If something Tahir comes into contact with something Najis, and one of these two things is wetter than the other and the wetness of one reaches the other, then that Tahir thing becomes Najis.

However, if a person does not know if something Tahir has become Najis or not, then it should be considered as being Tahir, and research and investigation is not necessary, even if one is able to ascertain whether the thing is Tahir or Najis.