One is a Snail Ten is a Crab. It's a "counting by feet" book in which even numbers are counted using creatures with even numbers of feet: humans, dogs, insects, spiders, etc... Odd numbers are counted with an even-footed creature, plus a snail. So...
1 is a snail.
2 is a person.
3 is a person and a snail.
4 is a dog.
5 is a dog and a snail. Etc.
The book continues in that fashion up to 10. After that, the text starts counting by 10s using a variety of animal feet.
P.S. Yes, that is my foot. It's the price for making this a "pin-able" post.