Just like in the fairy tale, this favorite lunchbox fruit can be a little poisonous. While the fruit itself is perfectly safe (though washing before eating is highly recommended), the seeds in the fruit can contain trace amounts of poison.
Apple seeds contain cyanogenic glycosides which turn into cyanide through an enzymatic process. Most people stop eating their apples at the core and never get to the seeds, making this less of a concern.
However, occasionally seeds do get chewed and ingested and while one or two won’t cause any problems, it isn’t recommended to have them as a snack!