Across the UK, 720 million eggs are ending up straight in the bin every year — and it’s costing Britain about £139 million annually, according to research released on Tuesday. 

Perhaps individually, it doesn’t sound like the biggest deal. But it’s all contributing to the estimated 10 million tonnes of food waste generated in the UK every year, which is costing about £20 billion every year. 

The reason so many eggs are being thrown away every year, according to consumer research from the company behind the food waste app Too Good to Go, is because a lot of people are being too cautious about best-before dates.

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Some 29% of Britons, according to the research, throw out the eggs after the best-before date has passed. But eggs can reportedly be safely eaten even after this date.

According to European Union law, reports the Guardian, eggs can be eaten up until 28 days from when they were laid. 

So Too Good to Go has launched a new social media challenge — all to get people talking about eggs and whether or not they really need to go in the bin. 

Did you know that over 720 million eggs go to waste every year in the UK? 😲 Try the #GoodEggChallenge for yourself and prove that just because an egg is past it’s ‘best before’ date it can still be an egg-cellent choice for breakfast, lunch or dinner! 🥚https://t.co/oVY8xFv3fU

— Too Good To Go (@TooGoodToGo_UK) April 9, 2019

It’s called the #GoodEggChallenge, and it’s raising awareness about a simple trick that you can do with your eggs to know for sure whether they’re actually off.

All you do is put the egg in a bowl of cold water. If the egg sinks and lies flat on its side, it’s very fresh. If it sinks and stands on one end, it’s less fresh but still good to eat.

When they float, that’s when to avoid them, as it’s no longer fresh enough to eat. 

The science behind it is essentially that an egg starts to decompose when it's going off, according to education resource ThoughtCo, and that decomposition gives off gases. A gas bubble starts to form inside the shell, and that makes it float. 

“If you’ve been throwing your eggs in the bin based on the dates on the box, you’ve probably been wasting perfectly good food,” said Jamie Crummie, the co-founder of Too Good to Go. 

“Food waste is a huge problem — a third of all food produced globally is wasted,” he added. “Small changes from each of us can make a big difference.”