Why does a mongoose die if bitten by a poisonous snake but can kill and eat the same snake and live?

When the mongoose eats and kills the snake, it knows to stay away from it's head, (Where the venom sack's are), and eat the rest of the body where none of the venom is. If the mongoose is bitten by the snake though, the snake will inject the venom through it's fangs, from the venom sacks, thus killing the mongoose.