Rare collectible coins can be worth far more than their face value - and the rarest 50p design regularly sells for 160 times what it's worth. But which coins should you look out for in your change?
Which? has rounded up the 50p coins that fetch the highest premiums from collectors, the factors that make a coin valuable and how to avoid being scammed.
There have been a number of special 50p collections released over the past few years, including designs featuring Beatrix Potter characters, Olympic Games sports and famous Brits, such as Sir Isaac Newton.
It takes into account the number of coins that were released into circulation, how many of each design are listed as 'collected' by its members, and the number of times a design has been requested as a swap, indicating the level of demand.
The graphic below shows the 10 rarest 50p coins - according to the Change Checker scarcity index - and the average amount they sell for on Ebay. This value has been found by taking an average from the five most recent sales.
While it's not the only factor, an indicator of whether a coin is likely to be valuable is how many are in circulation - the harder it is for collectors to find one, the more they may be willing to pay.
The mintage of a coin, meaning the number of copies issued by the Royal Mint, can be an indicator of how rare it is.
The graph below shows the 50p coins with the lowest mintages, as compiled by Change Checker.
It correlates that the 50p with the lowest mintage - the 2009 Kew Gardens coin - is consistently the most valuable.
But in some cases, coins fetch a premium even if the mintage is fairly high.
For instance, the Jemima Puddle-Duck 50p (part of the Beatrix Potter collection), regularly sells for £11, despite having a mintage 10 times that of the Kew Gardens design.
In these cases, coin enthusiasts may be paying above the odds to complete their collections, or because a design is particularly beloved.
Another factor is whether coins are hoarded in private collections. Some designs have a sentimental value, so that even non-collectors may hang on to them - making it difficult to get your hands on one, even if many were released.
Finally, keep in mind that most collectors prize 'mint condition' finds, meaning they are free from scratches, marks or even uncirculated. If your coin looks worse for wear, it's value could significantly drop off.
Due to the fact that large sums of money can be made from rare coins, the hobby has also unfortunately been linked with several scams.
Remember that coins are only worth whatever a buyer is willing to pay for them - even if you believe you have secured a rare coin, you may not be able to sell it on for a profit.