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You know the drill—someone brings out a birthday cake blazing with candles and sets it in front of you. So, you do the normal thing: you smile awkwardly while people sing you “Happy Birthday,” and then you blow out the candles.

You've probably done that since before you can remember. But if you think about it, that tradition—blowing on the cake that everyone's about to eat—is kind of gross. But just how gross is it to blow out birthday candles?

Luckily, a group of scientists at Clemson University also had this question a few years ago. The lead author was a food safety professor who was an expert in topics like the five-second rule, double-dipping, and how to throw a wild science party. To get to the bottom of this question, the researchers made a fake cake out of styrofoam and covered it with icing.

Then they stuck in some candles. Even though the cake was a lie, they wanted to simulate an actual party as closely as they could, especially to make sure that whoever was blowing out the candles would be salivating. So they had a pizza party.

You know, for science. After the pizza had stimulated their salivary glands, they lit the candles and blew them out. But that simulated party wasn't over yet!

Next came time to count the bacteria. Which is my favorite party game. So they diluted that spittle-covered icing and put it in Petri dishes.

Each colony that blossomed represented one bacterial cell from the icing. It's not a perfect method because some bacteria can scoot by uncounted, but it's a decent baseline. And their findings weren't totally surprising—especially if you ever had your suspicions about birthday cake.

On average, blown-on icing had about 15 times more bacteria than untouched cake. But what was odd was that the number of bacteria varied wildly by person. Some people showered the cake with more than 100 times more bacteria compared to an un-blown-on cake.

So the real message here might be to choose your friends wisely. In reality, though, many of the bacteria in our mouths are helpful, doing things like helping fend off infection. And a lot of them are harmless—they just live there!

The lead author figured that even if you add up the probability of getting sick over years and years of birthday parties, the chances are probably super tiny. The exception might be if the candle-blower is sick to begin with. Strep throat, the flu, and lots of respiratory illnesses get around on airborne particles, so it's at least worth being cautious.

Maybe you have a friend who's weird about birthday cakes. Or maybe you saw that video of a certain politician who went viral for taking the candles off his cake and blowing them out one by one. It's not the worst idea.

But common sense is a pretty good guide here. If blowing out candles was really risky, a lot more people would get sick after birthday parties. So go ahead and make a wish.

If all this talk of cake is making you hungry, maybe you'd like to try your hand at baking. Skillshare has tons of courses that could help you get started. For example, author and food blogger Sarah Graham hosts the course “The Art Of

Baking: A Beginner's Guide,” which will give you the tools you need to get started in the kitchen. She covers all the basics of cakes, breads, and pastries—and even a bit about the science of baking. Skillshare offers more than 25,000 courses in all, in topics from design to business and more. Premium subscribers get access to every single course, and an annual subscription costs less than $10 a month.

And the first 500 SciShow subscribers to use the link in the description will get a 2-month free trial. So you can check it out and see if Skillshare is right for you! [OUTRO ♪].