You shouldn't eat apple seeds because they are crammed with cyanide.
Behind the Legend
Apple seeds do indeed contain cyanide, just as bananas are full of arsenic and deadly nightshade contains hemlock. In fact, those who have been so unlucky to have been bitten on the tongue by a cobra maintain that the serpent's venom tastes distinctly like apple pie.
Fortunately, the poison in apple seeds exists in such a small amount that you would have to eat a very large number of the seeds (two or more within an hour for an average-size male) to have even an even chance of a painful, irreversible, slide toward death.
A far greater danger is found in watermelon seeds which, if swallowed, will take root in the small intestine and grow into full-fledged watermelon plants, culminating in a horrible, fruity explosion.