One of the most famous psycholinguistic tasks to have ever been done with children is the Wug Test.
First catch your child (aged about 4 for maximum effect), then show them a picture of a made-up animal-like object. Tell them, “This is a wug.”
Then show the child a picture of two of those objects, and say, “Now there are two of them. There are two _____”
If the child knows how to produce regular plurals in English, they should reply, “two wugs.”
So far, so obvious – but there’s more to it than that. In English, there are at least three ways of pronouncing the plural marker which we write “-s” – either /s/, /z/, or /əz/, depending on the phonological properties of the word you’re attaching it to.
If the word ends with a voiceless consonant, like cat, you use the /s/ version of the plural marker: /kats/.
If the word ends with a voiced sound (consonant or vowel), like dog or bee, you use the /z/ version: /dɔgz/, /biz/.
If the word ends with a sibilant, like horse, you use the /əz/ version: /hɔɹsəz/.
The different versions are called allomorphs – alternative forms of the one morpheme – and as you can see, the choice of which particular allomorph to use is rule-governed (or at any rate, it can be described in terms of fairly straightforward rules).
In the quest to find when and how children acquire this rule, Jean Berko Gleason came to fame with the wug test. As it explains in the Wikipedia article, it takes till about 5 or 6 years for children to master all the different aspects of this allomorphy – but the default /z/ form is usually acquired by the age of about 4. Meanwhile, the endearing little wug itself has taken on a life of its own and acquired an iconic status in the child language acquisition research community.
And in fact, so much so, that wug-based merchandise has in fact been recently made available from CafePress.com. You can get wug t-shirts, bags, aprons, and even a wug mug! It’s so exciting, and all in time for you know what pagan festival as well.
So if you were, in fact, wondering what to get me … well, first of all try Present Aid or Oxfam Unwrapped, but if you were still feeling generous after that, I really don’t feel my life would be complete without at least, let’s say, a fridge magnet to accompany Nelson Mandela’s ‘great generation’ quote and the obligatory humorous ‘sometimes I wake up grumpy’ one. The perfect gift solution, as the saying goes, for the language acquisitionist in your life.