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Right-click a dog while holding a biscuit. If the dog isn't tamed it will eat the biscuit. It will require 1-4 biscuits to tame the dog, However it's extremely rare to require over 4.
If your dog is already tamed while trying to give it a biscuit, your player will automatically shove the biscuit up the dog's anal cavity. Doing this will give the dog internal bleeding and it will lose 2-4 hearts. (JOKE)