If you ever find a pocket of flour in a baked matza — do not put it into gravy. If you do, it will turn chometz and the food will become ossur b’hano’ah. (Siman463 — MB8)
Saliva from humans and farm animals can cause grain flour to leaven. Steam can cause grain flour to leaven. (Siman466 — ShA1,2 — MB7)
The poskim argue about dampness that ‘sweats’ from walls. Lechatchila, do not use flour that became wet with such moisture. (ShA3 — MB8)
Using a gentile’s chometz medicines to heal illness or wounds is allowed if there is no alternative way. You do not own the chometz and the issur hana’a is only mid’rabbonon. (MB1)
If there is danger to life you are allowed to benefit from chometz and even eat it if there is a chance it will help. If there is no danger to life even a crumb is ossur. (MB2 — ShaH6)
The following fluids can leaven just like water: (1) dew; (2) urine; (3) fluids from the nose, mouth, eyes, and ears; (4) runny bowels. (ShA5 — MB24,27)
The following fluids do NOT cause chometz: blood, milk, fat, bile, sweat, excrement. (ShA5 — MB25,26,27)
