It's the unquestioned conclusion to a party for birthday boys and girls, big or small.
But a new study has shown that blowing out the candles on a birthday cake leads to 15 times more bacteria being present on the icing than there was pre-puff.
One can only imagine the bacteria levels sky-rocket if an over-enthusiastic toddler is doing the honours.
Researchers from Clemson University in South Carolina made a fake birthday cake by icing a piece of foil and placing it on top of a Styrofoam base.
Next they tested the amount of bacteria on the icing before and after the candles had been blown out, and the results were pretty gross. They discovered that on average there was 15 times more bacteria on the icing after the candles had been blown out.
The next finding was equally unappetising. After the candles had been blown out the variation of bacteria on the cake was 100 times greater.
But, don't bin the candles just yet - there was some good news. Despite the bacteria overload, researchers said most were harmless.
"It's not a big health concern in my perspective," co-author of the study Paul Dawson told The Atlantic.
"In reality if you did this 100,000 times, then the chance of getting sick would be very minimal."
He's been conducting a series of studies around food safety, and the spread of bacteria, with his undergraduate students, including the five-second rule, double dipping and sharing popcorn.
For this study, he wanted to ensure the students felt like they were at a party so they ate pizza before conducting the experiment.
"We also wanted to simulate a birthday party," the professor of food safety at the university said.
"We thought it might help the salivary glands get going."
And it did, but the amount of bacteria spread varied from person-to-person.
"Some people blow on the cake and they don't transfer any bacteria," he said.
"Whereas you have one or two people who really for whatever reason… transfer a lot of bacteria."
Little or a lot, he said if someone was sick it would be best to avoid a cake if they'd blown out the candles. Just in case.