A new £1 coin has gone into circulation in the U.K. — and Brits are trying to guess what the top-secret security measure in the piece of money is.

The hidden change in the revamped coin, introduced on Tuesday, is meant to baffle counterfeiters, according to the Royal Mint. About one in 30 of the existing £1 coins is fake, it said.

The “highly secure” update has a bunch of other features designed to make it harder to copy. For a start, it’s thinner, lighter and slightly bigger than the round coin that replaced the £1 note in 1983. It’s 12-sided and made of two metals, not one. There’s tiny lettering around the rim on both sides, with one showing the year of production.

And most high-tech of all, there’s a hologram in the image that shows “£” or “1”, depending on which way you squint at it.

But it’s the secret sauce, known only to the Royal Mint, that has got people wondering as to what exactly the tech is.

Others are bemused about how they can tell a new coin is not genuine, given they can’t use the mystery feature.

By coincidence, the new £1 has come out a day before Prime Minister Theresa May is expected to invoke Article 50 and begin the process to take the U.K. out of the European Union. The slide in the value of sterling since the Brexit vote in June has prompted some bitter comments.

Americans with a pile of leftover vacation cash should take note: The last day to trade in the old coin for the new version is Oct. 15.