This is the one you've been waiting for, the one that gives you an excuse to buy a Grimm's rainbow.
Seriation is the idea that you can organise objects according to characteristics such as height or length.
Like a lot of the concepts we cover in this series, you can try to teach it explicitly, using Montessori knobbed cylinders, for example. But it's much better to let children figure it out for themselves.
Children under one will spend many happy hours trying to post an object into an opening. They are starting to understand bigger and smaller.
Then it's time for small, medium and large. Read Goldilocks or The Three Billy Goats Gruff. Play with nested toys like bowls, boxes or Russian dolls. Arrange toy figures in height order. Build steps or pyramids with blocks.
After that you're ready to tackle ordinal numbers, 1st, 2nd, 3rd.
And there's no need to buy any toys. A rainbow is nice, but empty yoghurt pots of various sizes will do just as well.
You're probably familiar with the expression 'baby knows best'. It applies to pre-schoolers, too. An environment rich in experiences and the freedom to experiment and make a mess is all they need.
Let them play.
Here's the rest of the series:
Encourage open-ended play for creativity and focus
Understanding schema play in toddlers
Independent play and why it matters
Insider Guide: Small World Play and Language
Tell Almost Any Story with Just a Handful of Figures
A simple guide to choosing the right toy