One of the very first sensorial materials introduced in a Montessori classroom are the Cylinder Blocks. They are made of wood, with 10 cylinders cut out of each block. In one block, the cylinders are all the same height but decline in width from thick to thin. In another block, the cylinders are all the same width but decline in height from tall to short (this is the hardest block). A third block declines in both height and width (Tall/Thick down to Short/Thin) while the last declines in height while increasing in width (Tall/Thin to Short/Thick).
I purchased these secondhand from a local homeschooling mom just before DJ's second birthday. They are meant for the preschool years from 3 - 6 years old, but he was able to enjoy them one at a time as puzzles early on. Then his interest declined and he didn't choose them for several months. A few weeks ago, right around his 2.5 year half-birthday, there was a whirlwind of activity. One block at a time is way too easy and he much prefers two blocks. Lately he has been insisting on bringing out 3 or 4 at a time, but then gets overwhelmed and won't complete them, so he still has more to learn.
The goal of this activity is for the child to learn to discern sizes and I enjoy watching as DJ tests out various options, a very small cylinder in a large hole, a tall cylinder in a shallow hole, or he'll slide a cylinder from one hole to the next to figure out which it belongs in. I've even seen him leave a cylinder in the wrong hole. That is when the self-correcting nature of this material comes into play. He can't place the 10th cylinder when any one of the other 9 is wrong, which forces him to check his work. There is no need for me to ever say, "No, not that one".
Here is a video that I took of him recently. The video is 1.5 minutes long. I apologize for the black border. I have since learned that I have turn to my iPhone landscape to take video. So future videos should be better. Enjoy!