This is more of a curiosity question than trying to answer a problem, but it is something I am sure that many parents have wondered in the past.
I notice that my kid (almost 2) will, without fail, toss aside any food he's not completely familiar with on his plate, be it half-grapes, strawberry bits, cut up banana, or really anything that isn't on bread, cheerios, or shaped like a dinosaur.
But, when I take him outside to play, he will eagerly pull up any type of grass and shove it in his mouth, will gladly pry moss off of a tree and eat that, will literally put dirt in his mouth, and will even try to eat gravel. Most frustratingly, he will grab wood chips off the playground and put those in his mouth, which could potentially give him splinters in his mouth.
What is it about the random objects he finds on the ground outdoors that compels him so much to eat them at all times, that does not give him any kind of similar compulsion when put in front of him to try as a food?