Eat oysters only in months whose names contain the letter R. That old rule of thumb lingers, seemingly as fundamental as “30 days hath September.”

And yet there I was in late May, clearly not an “R” month, at Glidden Point farm in Edgecomb, Maine, with three oysters, a knife and a tour guide determined to teach me how to shuck them without losing a thumb.

I wedged my knife into the hinge of the shellfish, then realized in horror that this was a living thing that I was about to savage. My guide, Althea Pendur-Thorne, read my mind.