Wow, this topic seems to be getting people angry lol. Anyway, I'm also a psyc major (actually reading my textbook right now about children's memory capabilities), so I thought I'd put my 2 cents in as well. It seems that very young children do not seem to have the ability to understand symbolism. A great article titled "The Credible Shrinking Room: Very Young Children's Performance With Symbolic and Nonsymbalic Relations" taken from Psychology Today, 1997, 8, addresses specifically this issue. An object was hidden in a miniature model of a room and the children were told to find the object in the larger room. Anyway, they found that children aged 2.5 years were unable to draw a relation betwen the miniaturized model of a room and the actual room itself, upholding their theory that very young children are unable to create "dual representations." Interestingly, when the children were told that the small room was actually the larger room and had been put in a "magic shrinking machine," 80% of the children had no problems finding the hidden object. It was not until age 5 I believe that children were able to construct the "dual representation."
Also, going along with psychological theory, one of Vygotsky's theories was that of the zone of proximal development-"the region of sensitivity for learning characterized by the difference between the developmental level of which the child is capable when working alone and the level she is capable of reaching with the aid of a more skilled partner." So, basically all children have a level at which they max out no matter how much you push them. If they do not already have above average intelligence, you cannot push them to reach a 1500 on their SAT no matter how hard you try. I know everyone has been posting about "dumbing down the schools" which to a certain extent is true. But if all lessons were structured around the most intelligent children, the children lowest in intelligence would lag behind. If you want to campaign for more gifted classes, etc, I'm all for it, but I don't think that the slower children should be punished with lessons way out of their understandings.
p.s. before I get all kinds of posts about how your child understands symbols etc, remember that there are always exceptions to everything.
And we'll keep our Christmas trees and stick the holiday trees where the sun don't shine. 
