@MonicaCellio He derives that (dry) sand isn't a problem from Shabbos 39a (איכא בינייהו עפר תיחוח). See Rabbeinu Tam cited in Tosafos there who is of the opinion that this only refers to sand that is so loose that an egg would basically sink into the sand of its own accord if it was place there.
@MonicaCellio Rabbeinu Tam's reason is that such sand is already so loose that it is impossible to practically loosen it further.
@MonicaCellio R' Ribiat mentions another reason in a footnote, namely that dry sand is not fitting for planting (based on P'sachim 47b).
@IsaacMoses Welcome BabyMoses! He may be interested in in our 30+ questions tagged baby-infant or our nearly 100 tagged brit-milah. I look forward to seeing him around!
@DoubleAA, @Fred - thanks for the quick help with that no-longer-a-duplicate question. Funnily enough, the current question is actually the one I was working toward, but having failed to find the other one I found myself questioning my assumptions -- maybe that's not why we do it, etc. So I asked that first with the idea that I'd ask this as a follow-on if applicable. Thanks for saving me a step.
@DoubleAA Thanks for the warm welcome! You actually caught him a few days too late, since he was apparently holding in those and many other sugyot then.
@IsaacMoses I'm confident that by the time he's allowed to have his own account instead of posting through his father, he'll have reacquired enough to start answering questions.
