For food that has been plucked from the ground or pick from a tree to become susceptible to tumah, it must first come into contact with one of the seven liquids, one of which being water -- this is referred to as hechsher. The first Mishnah teaches that this contact must be in concert with the wishes of the owner (le'ratzon) and/or1 the water must be detached from its source willingly.
The Mishnah (1:5) record the opinion of R' Yossi, that if one had wet hair and wrung it out with a towel, then the water that leaves the hair can affect hechsher while the water that remains cannot.
The Bartenura explains the person's hair was wet due to rain and the person wrung their hair out to remove that water.
Rashi (Keritut 15b s.v. Hasochet) however explains that the person's hair was wet because of taking a bath.
The Tosfot (s.v. Hamochek) however finds Rashi's explanation difficult. Despite the fact that one does not want their hair wet after washing, they certain wanted it in the beginning, when they washed. Consequently, this would be considered "techilato le'ratzon". It was initially detached willing, and that would be sufficient for that water to affect hechsher. That would mean that even the water that remained behind, that was not wrung out, should affect hechsher. The Tosfot therefore cites the Ri, who explains like the Bartenura that the individual's hair became wet due to rain. In that case, it was never le'ratzon and only become so when successfully removed from the hair.
The Aruch La'Ner's (s.v. U'Ma) second answer is that in this case, the individual simply did not want to get their hair wet. He suggests that the Mishnah discussing the hair, rather than discussing the water on his body more generally, supports this idea that there is something different about the hair -- he never wanted it to get wet.
According to this answer, there does not really appear to be any substantial difference between Rashi and the Bartenura. They appear to be simply providing two different possibilities where the individual did not want their hair to get wet from the beginning.
In his first answer however, the Aruch LaNer explains that the Mishnah cited in the Gemara on which Rashi comments is different to our Mishnah. Firstly, it omits that it is R' Yossi's opinion. Furthermore, it simply states that the water remaining "in the hair" cannot affect hechsher without providing the rationale presented in our Mishnah that the person wanted all the water to leave. The Aruch LaNer therefore reasons, that Rashi understands that that which was presented in the Gemara was either a Beraita or a different Mishnah. In other words, the case in the Beraita is different, in order to teach a different law. The reason the water remaining cannot affect hechsher is not because it was not detached le'ratzon. Instead, the reason is since it is absorbed, it is considered annulled; it loses its significance and can no longer affect hechsher. In other words, even if it was indeed detached le'ratzon, and could have affected hechsher, now that it is absorbed in the hair, it loses its ability to do so. Based on this explanation, we can understand why Rashi did not explain the case like the Bartenura (that the hair became wet in the rain). Had that indeed been the case, one would not have been able to deduce this new law. Consequently, Rashi explained that the individual bathed, and his hair did become wet le'ratzon. Nevertheless, since it is absorbed, it cannot affect hechsher.
1 See volume 6 issue 61, "Hechsher -- willingly".
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