For centuries, tea drinkers in China have used the pee-pee boy to measure the temperature of water.
Discerning tea drinkers will tell you that water temperature is important when brewing tea. But how do you know when it’s hot enough?
In China, there is a ceramic toy called a pee-pee boy, with a novel way of letting you know when your water is hot enough for brewing tea.
Pour water over the little guy, and if he starts to “pee,” you’re good to go.
A pee-pee boy is what’s called a tea pet. Tea pets are ceramic toys that react to hot water. Tea drinkers often have them on the table for entertainment.
“In the past, there were no tea pets on the tea table,” says Huang Yun, an artist who makes tea pets. “There was just a pot and cup. But as living standards improved, people started caring about aesthetics, and that’s how tea pets came about.”
Most tea pets are made with zisha, an iron-rich clay found in Yixing, China. The clay here is also used to create the famous Yixing teapot, which is said to absorb the flavor of tea over time.
(Read more: The teapot that makes tea taste better)
Tea pets are often kept for years, and owners shower them with love (and boiling water) until the ceramic turns a greenish-brown hue.
The pee-pee boy is not the only tea pet. There are also animals that change color and statues with moving parts, such as a toad with a coin that starts ringing when water is poured over it.
But by far the most popular tea pet is the pee-pee boy, says Huang.
The science behind the pee-pee boy
Because of the way it reacts to hot water, some believe the pee-pee boy could be the world’s oldest thermometer.
A paper written by two researchers at Iowa State University details the thermodynamics at play.
Tea drinkers first douse the pee-pee boy with hot water and then submerge it in room temperature water to fill it up.
By heating up the pee-pee boy, the air molecules inside expand. When it’s dunked in cooler water, the air molecules shrink, reducing the air pressure inside and forcing water to enter through the pee hole.
(Read more: How to perform a traditional Chinese tea ceremony)
The water stays inside until warmer water—such as that used to brew tea—is poured over the pee-pee boy.
That causes the air molecules inside to expand again, forcing water to jet out of the pee hole.
This principle of thermodynamics is also what powers a steam engine.
Traditionally, tea drinkers would gauge the temperature of water poured on the pee-pee boy by the distance of the stream—the longer the pee, the hotter the water, which is why some people call it the world’s oldest thermometer.
Nowadays, though, most people just use it to add a little more fun to tea time.