I admit I was taught to present the pink tower, broad stair, and red rods before introducting the knobbed cylinders. I found with my own children that the easiest cylinder blocks, the ones that vary in 3 dimensions, can be introduced to two year olds. My children did the 2 easiests cylinder blocks before the tower and stairs. The cylinder blocks have a great control of error, the cylinders need to fit together properly before the lesson can be completed. The socket helps a child figure out which cylinder fits by trial and error. Later, after doing the tower and stairs, my children completed all four cylinder blocks with the extensions.