A four year old child sits in front of a heap of wooden blocks and tries to stand them in a straight line in order of size. There is no doubt that he understands the task, for he can distinguish between correct and incorrect solutions. Yet no matter how he arranges and rearranges the blocks, they seem obstinately to refuse to be aligned into the desired staircase­ like pattern. A year later he does much better: he can seriate six blocks differing in length by about an inch. But faced with eight of the blocks, or with six blocks differing by only half an inch, he is again unable to complete the task. He manages to build an ordered line, but leaves out one or two of the blocks: or perhaps he organises the blocks into two distinct lines and then finds himself unable to combine them. A year later still and he is now an accomplished performer. Presented with any seriation task involving the wooden blocks he carries it out smoothly, confidently, and correctly [1].