The Temperature You Can Hear?

Imagine you have in front of you a mug of water. You look at it, and you can’t tell if it’s hot or cold. You smell it — same problem. But when you touch the water, you know instantly whether it’s good for tea or, well, not.

But there’s also another way to tell: with your ears.

Let’s try something new today — we’re going to do a science experiment. The YouTube video below is about 27 seconds long, but I’m going to ask you to pause after about 15 seconds. (Don’t worry: the person who made the video is also going to ask you to pause at that point, too. So you won’t forget!) After watching the first fifteen seconds, please take the one-question poll below.

Here’s the video:

And here’s the poll:

[Okay, I haven’t figured out how to get beehiiv’s polls to embed on the newsletter’s archives. Check your inbox instead!]

Here’s a prediction: the vast majority of you will get the question right. (I’ll share the right answer at the end of this paragraph.) Because as it turns out, most people can tell the difference between the sound of hot and cold water being poured. In 2014, per NPR, a sonic marketing agency was investigating “how they might use sound design to make better beverage ads” so “they played sounds of hot and cold water being poured into glasses and asked people to guess: hot or cold? The results were kind of insane. Ninety-six percent of people can tell the difference between hot and cold, just by the sound.” And if you guessed that the hot water was the second one poured in the video above, you’re one of those 96%.

It’s a great example of people learning without trying — as Science Alert notes, “What’s happened is your brain has subconsciously learned to pick up on the differences between the two sounds from all the hot and cold drinks you’ve heard being poured throughout your life. You may not have thought about the variations, but they’re there.” But that’s only half the story. Why do hot and cold water make different sounds when poured anyway?

For years, the best guess science had to offer was based on viscosity — as NPR noted, “cold water is more viscous than hot water, because the molecules are wiggling less rapidly, so they are effectively stickier.” But that’s probably not what’s casing the difference in the sound. In March 2024, Xiaotian Bi, who had recently earned a Ph.D. in chemical engineering from Tsinghua University in Beijing, published a paper on the topic and came to a different conclusion. As the New York Times summarized, Dr. Bi concluded that “it’s all about the size of the bubbles that form during pouring, he says, and this insight may have implications for how we enjoy everyday food and drink.” Pouring hot water into a cup formed bubbles in the 5 to 10 millimeter range, while cold water produced bubbles that were 80% smaller, in the 1 to 2 millimeter range.

But of course, us humans can’t detect this difference with our eyes — Dr. Bi needed to use high-resolution photography. The good news is we don’t need to. When the bubbles pop, they make different sounds, and we can detect those differences with our ears.

Well, most of us can.

Bonus fact: Radio waves travel at the speed of light — about 300 million meters per second. Sound waves travel (unsurprisingly) at the speed of sound — about 340 meters per second. As a result, if conditions are right, you can hear something on the radio before you hear it in real life, even if you’re standing really close to the source of the sound. Want to try this in real life? The BBC, accidentally, makes that easy. The Great Clock of Westminster in London, better known as “Big Ben,” chimes every hour. The BBC broadcasts the 6 PM and midnight chimings direct from the source, with a microphone positioned right near the bells. As Londonist demonstrates in this YouTube video, if you’re standing on Westminster Bridge — just outside Big Ben — and have an FM radio tuned to the station that plays those chimes, the sound of the bells will hit your radio a split second before those same chimes hit your ears directly.

From the Archives: The Surprising Story Behind the Sound of Sneezes: It’s a habit.

Related: An electric tea kettle, in case you want to do the water sound experiment at home.