One of the oldest tea accessories to grace the Chinese tea table is the tea pet, and one of its earliest functions was as a thermometer.

Taking tea gongfu style, you might come face to face with a lucky frog. Jin Chan is a rotund, three-legged bullfrog who sits on a pile of Chinese coins. He is decorated with seven spots on his back – representing the Beidou, or Northern Dipper, constellation – and often has a coin in his mouth. In feng shui, he is said to attract wealth and repel bad luck.

Jin Chan is a popular type of tea pet, a figurine that sits on a tea tray and shares your tea session. Gongfu practitioners will pour wastewater and discarded tea over the pet to ‘feed’ it and, over time, it will develop a patina representing all the sharing and caring you’ve offered through the years. Tea pets date back to the Song Dynasty (960–1279) and were originally made of leftover clay used for pottery in areas such as Yixing in Jiangsu Province. Common motifs are Chinese zodiac animals, fruits and vegetables. One novel tea pet is the pee pee boy, a large-headed fellow whose member has a hole for shooting water across your tray.

Amusing as this is, pee pee boy actually has a practical purpose: he’s an early thermometer. Before the tea session, the hollow figure is submerged in room temperature water. Once the kettle heats up, hot water poured over the boy causes him to pee – the stronger the jet, the hotter the water.

How? Thermal expansion. When the air pocket inside the tea pet’s head is suddenly heated up by the hot water, it pushes the water from his earlier submersion out. If you’re after a certain temperature for steeping your tea, this fountain of youth can tell you when the water’s ready.

What’s interesting is that pee pee boy may merely be a later iteration of thermometers from the Tang Dynasty (618–907 CE); archaeologists have uncovered frog-shaped ornaments with holes in them that are said to spew water. All this pre-dates Galileo’s thermoscope by several centuries – and all for a properly steeped pot of tea.

Read more: Lee, V. and Attinger, D. Thermodynamics and historical relevance of a jetting thermometer made of Chinese zisha ceramic. Sci. Rep. 6, 28609; doi: 10.1038/srep28609 (2016).