Chagigah, Chapter 2, Mishnah 6. This mishnah teaches a fundamental principle: when a person comes to purify himself, the act of immersion alone is not enough. If he goes down to the mikvah for a particular level of purity, he must have that exact level in mind at the time of his immersion.
The levels of purity in the mishnah:
"Hatovel l'chulin v'huchzak l'chulin - assur l'ma'aser" - one who immerses in a mikvah so that he may eat unconsecrated food (chulin) in purity, and his intent was for chulin alone, may not eat ma'aser sheni until he immerses for the sake of eating ma'aser sheni, which is a higher level.
"Taval l'ma'aser v'huchzak l'ma'aser - assur l'Terumah" - one who immerses for the sake of eating ma'aser sheni has the status of one who is pure for ma'aser, and nevertheless he still may not eat Terumah, if he is a kohen.
"Taval l'Terumah v'huchzak l'Terumah - assur l'kodesh" - one who immerses for the sake of eating Terumah has the status of one who is pure for Terumah, but he still may not eat kodashim, meaning sacrificial food.
"Taval l'kodesh v'huchzak l'kodesh - assur l'chatas" - here his intent at the time of his immersion and purification was to be considered pure for sacrificial food, and he indeed holds that status; even so, he still may not handle the mei chatas, the sprinkling waters that purify from the tumah of a corpse.
The principle that emerges from this:
"Tovel l'chamur - mutar l'kal" - one who immerses for the stricter level is automatically permitted in the level that is more lenient than it, since the lenient level is included within the stricter one.
"Taval v'lo huchzak":
One who immersed without any intention at all of establishing himself at a particular level, but rather went down to immerse merely to wash, is considered as though he had not immersed, since immersion requires intent. Nevertheless, his immersion is effective for chulin, because eating chulin in purity does not require intent; only the higher levels, ma'aser sheni and above, require intent.
In summary: in this mishnah we learned that an immersion is measured by the intent with which it was performed: one who immerses for chulin may not eat ma'aser, one who immerses for ma'aser may not eat Terumah, one who immerses for Terumah may not eat kodesh, and one who immerses for kodesh may not handle chatas. The principle is that one who immerses for the stricter level is permitted in the more lenient one, and that one who immerses with no intent at all is not considered to have immersed, except with regard to chulin, which does not require intent.