Murphy's law?

Why is it that babies will put everything--and I mean every thing (see previous post)--right into their mouths for a little chew fest, but as soon as you get a spoon with yummy orange baby food close to their mouth, they refuse to open up?!
Is this because they instinctively know that the food is good for them, and therefore already hate it? Or maybe, since they know Mommy wants them to eat it, it stops looking tasty?
I don't know why, and I certainly can't explain my daughter's actions. She couldn't possibly remember that I tried to stuff the same orange goo down her throat yesterday, and it tasted like that soggy cardboard box she was chewing thru earlier that morning. What could she be thinking?
"Mom, when I want the whole 'soggy cardboard' taste in my mouth, I will just crawl over and have a chew on the box in the other room. I don't need you cramming this down my throat. Oh, and quit talking gibberish. I may be an infant, but do I look stupid? No, I think not. Now if you're not going to give me something good to eat, like some clods of dirt, or small twigs, just put me down before things get ugly."