Comment by whyenot

As Alex noted, apple seeds contain cyanide, or to be more accurate, cyanogenic glycosides. When the tissue of most apple seeds is damaged, an enzyme cleaves the sugar-cyanide bond, releasing hydrogen cyanide gas. You can test this yourself, by crushing some apple seeds and smelling the mashed seeds. It will smell of almonds, which is the smell of hydrogen cyanide gas.

I created a lab for intro biology around testing for HCN in various plant tissues. The interesting thing is not all apple seeds actually seem to produce HCN. Granny Smith seeds seem to always produce it (and relatively speaking a lot), while some little lunch box sized Fuji apples I bought at a farmers market don't produce any at all.