Cylinder blocks

BabyEbi has been working with these for a few weeks now. He can do 2 blocks together so yesterday I tried him with three, seemed to be going OK but he suddenly got up half way through to get a drink, but he came back about 10 minutes later to finish it.
The knobbles on the top of the cylinder are to help stregthen the pincher muscles needed for writing. There are 4 blocks in the set, one has cylinders of the same diameter but different depth, another has the same depth but different diameters and the third starts with small diameter and small depth going to a large diameter and big depth the last block is the opposite, like the picture below.

All 4 blocks are different, though they look similar. Through experimentation the child finds out about their properties, how they differ in length and diameter. The cylinder blocks are self-correcting because, if a child puts a small cylinder in a bigger hole, then the other 9 cylinders will not fit back into the remaining 9 holes. Since the cylinders all fitted to begin with, it is easy for the child to see that there must be something wrong. In Montessori terms this is called an in-built ‘Control of Error.’ A lot of the Montessori equipment is self correcting so the child learns from his own mistakes.