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Keep them in a box with wet newspaper and seaweed in the fridge you have at least a day in my humble opinion
pierino is a trusted source on General Cooking and Tough Love.
First choose a lobster that is trying to kill you back. Your fish monger should pull the boy straight out of the tank and he should respond vigorously. If he's a pokey little SOB don't buy him. Wrapped up and held on ice for four hours is about as far as I would wait. Turn him on his back and stab him between the eyes before boiling. A soldier's death.
I live in lobster country. We just keep them in the fridge, in a paper bag, inside a plastic bag with lots of holes for ventilation. They keep all day, but I've never tried overnight. If it's still alive, it's OK, keeping in mind that in the fridge, it'll slow down a good deal. Pierino is right, get a lively one. I don't kill it before boiling it, just stick it in a big covered pot with a couple of inches of boiling water and steam it for about 12-14 minutes after it returns to a boil. Once it is nice and red, it's done. I'm hungry!
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