Niddah 6:1-2

Niddah 6:1

If the lower sign of maturity (pubic hair) appears before the upper sign (breast development), she can perform yibum or chalitzah. If the upper sign appears before the lower sign – though this isn’t possible – Rabbi Meir says she cannot perform yibum or chalitzah. The Sages say that she can perform yibum or chalitzah because they maintain that the lower sign can appear before the upper but the upper sign can’t appear before the lower.

Niddah 6:2

Similarly, any earthenware vessel that lets liquid in when submerged certainly allows it to flow out, though some vessels let it out without allowing it in. Any piece of a corpse that has a nail certainly has a bone but a piece with a bone doesn’t necessarily have a nail.