daf bit: Nazir 39

The mishna talks about someone who cuts his hair before his period of being a nazir is completed, saying he incurs a penalty. This leads to a discussion in the g'mara about how hair grows, because they're trying to figure out if the hair that was on his head at the time of his vow (which became consecrated) has been cut off. So does hair grow from the scalp or from the tips? The g'mara argues and then counter-argues for each of these possibilities, before finally resorting to this argument: when sheep are tithed every tenth one is marked with a bit of paint; the mishna providing this instruction would not have done so if that paint would soon be covered by new growth; therefore wool grows from the roots. An argument based on an explicit teaching of the mishna is more credible than one made from reasoning alone. The g'mara then observes that when old men dye their beards they grow white at the roots, so sheep's wool and men's hair both grow from the roots. (39a-b)

(Today's daf is 40.)