To boil crab, gather a large pot, a pair of long tongs, a preferred seasoning compatible with seafood and salt. Bring the pot to a boil, and add the crab to the water, cooking it until the shell turns red. This takes about 20 minutes for full crabs or Dungeness legs.
When adding water to the pot, do not fill it past the two-thirds mark, as this can prove dangerous when adding extra elements, particularly the crab. Once the water is added, bring it to a boil and add selected salt and seasonings. Once the boil arrives, add the crabs whole to the pot, slipping them in carefully to avoid splashing and burns.
When considering cooking time, make sure to begin counting not after the crab is added, but after the boil returns to the pot. After the crabs are finished cooking, it is imperative to cool them. One method is to simply remove the crabs from the water and place them on waiting clean, dry towels. A better option is to place them in a cool water bath, such as in the sink. This helps arrest the cooking process and keeps the crab meat from sticking undesirably inside the legs.