Have you ever watched a child play with blocks? Sometimes the younger ones are still learning about objects by touching or even tasting them. No matter how many times you tell them not to put it in their mouth your words are largely ignored. Sometimes they at least try to hide it by turning their back to you. Yet other times they just look at you with that lower lip quivering hoping that you will give in. Once they see that you are not going to let them do it they simply go back to building and try to slyly sneak peeks to see if you are watching.
They love to have you try to guess what they are building just knowing that you would never in a hundred years get it right. Their imaginations are vivid and they know exactly what they are trying to build. Attempting to create what is in their mind helps them develop more controlled motor skills. The are trying to figure out if that tall tower they built will stay up or crumble to the ground if they put just one more block on the top.
And just because they think it is funny they will build that tower over and over again just to watch it crash when they put that "one more block" on the top. But don't let anyone else put that block on there and make it fall or they will face the wrath of a two year old that didn't really want to share to begin with.
So get out those blocks for your child and stand back, they will have that bridge built in no time. Oh yeah, if you ask very nicely they may let you join in the fun.