A tevul yom (immersed, awaiting nightfall) touches terumah food. What degree does the terumah become?
Halacha 3: the tevul yom is a 2nd derivative; touching terumah food makes it a 3rd derivative, which disqualifies it for terumah.
Question 2
Can a person ever be considered a 3rd degree derivative of impurity (shlishi)?
Halacha 8: persons and vessels never become 3rd or 4th degree — neither by Torah law nor Rabbinic decree.
Question 3
In what case can liquids be considered 3rd degree (shlishi) derivatives?
Halacha 10: liquids touched by a tevul yom that are terumah become 3rd degree; if they are kodesh they become 4th degree. These are the only exceptions to liquids always being primary.
Question 4
The saliva of a tevul yom falls on a terumah loaf. Is the loaf disqualified?
Halacha 6: the tevul yom's saliva is like the liquids he touches — pure for ordinary items, 3rd degree only if the liquid itself is terumah. His saliva falling on a terumah loaf is pure because the saliva is like liquids touched by him — the loaf (not a liquid) remains pure.
Question 5
What is the maximum level of impurity a food can reach by Rabbinic decree?
Halacha 9: foods can become 3rd and 4th degree derivatives by Rabbinic decree — but the 5th degree does not exist. For ordinary food the maximum is 2nd; for terumah, 3rd; for kodesh, 4th.