What do you notice?
You should notice that your throat is vibrating or buzzing when you say zzzzz but not when you say sssss.
Linguists call this buzzing “voicing” - /s/ is voiceless (no buzzing), /z/ is voiced (buzzing). One way to remember is that the word “buzz” has zz in it.
Now let’s look at the sounds in the words that have different types of plurals.
We have “cat,” “top,” “bike,” and “blick” in the s group, and “dog,” “bird,” “cab,” and “wug” in the z group.
Put your hand on your throat and say the last sounds in each of these words. So tttttt for cat versus ggggg for dog, and so on. It’ll be a little bit harder because it’s hard to make a t sound without a vowel, but do as best you can.
You should notice that the t in “cat,” the p in “top,” and the k in “bike/blick” are all pronounced without buzzing (voiceless, like s), while the g in “dog/wug,” the d in “bird,” and the b in “cab” are all pronounced with buzzing (voiced, like z).
So all the words where the last sound is voiceless have s, the voiceless version of the plural, and all the words where the last sound is voiced have z, the voiced version of the plural. It’s almost like they match!
It’s more likely to have two voiced or two voiceless sounds together.
In fact, it’s exactly like they match.
There are really good reasons why it’s more likely to have two voiced or two voiceless sounds together, rather than one of each: it takes a little bit more time and effort to switch your vocal cords from vibrating to not vibrating, whereas it’s easier to just keep having them do whatever they were doing before.
Even without having any conscious awareness of what voicing is, you’re constantly paying attention to it because you don’t say “catzzz” or “dogsss,” and you’ll produce the matching plural even for new words like “wug.” In fact, the original wug test done by Boston University researcher Jean Berko Gleason showed that even fairly young children produce the matching plural for words they’ve never heard before. Which is pretty cool.
Patterns like this, where sounds that are close to each other become more similar to each other (the technical term is “assimilation”), show up in most (all?) of the world’s languages, because we’re all speaking with basically the same mouth and throat anatomy.
Bonus: What about words that end in s or z, like “glass” or “fuzz”? Adding s or z respectively gets us “glassssss” and “fuzzzzz,” which just sound like drawn-out versions of the same word. So we add a vowel in between the word and the plural marker to separate these sounds which are too similar. Very similar or identical sounds are also hard to pronounce close together.
The original “wug test” was done by Boston University professor Jean Berko Gleason and published in 1958.
This article was written by Gretchen McCulloch and originally appeared on her blog All Things Linguistic.