You handle them every day, and you probably imagine that you know your banknotes reasonably well.
But while there's every chance you know that Winston Churchill is on the new £5, Jane Austen on the new £10 and Charles Darwin on the old one - the notes are actually packed with messages and images - both clear and hidden - that we've never spotted.
See also: Is there a mistake on the new £10 note?
We reveal 15 quirky facts about the hidden images and codes on banknotes.
1. A new study, by FXTM, highlights that around the portrait of each famous person is imagery from their life. On the new £5 note, for example, Winston Churchill, appears in front of the Nobel Prize for Literature he won in 1953.
2. A more hidden bit of imagery on the £5 note is that Big Ben is showing the time of three o'clock. This represents the time in May 1940 when Churchill made his famous speech containing the phrase "I have nothing to offer but blood, toil, tears and sweat."
3. The £5 note also features a circular green foil patch with the word Blenheim written inside - reflecting the fact that Churchill was born in Blenheim Palace. The design and the foil are deliberately designed to fox counterfeiters.
4. Another hidden feature of the note appears around where the denomination is written - in this case the word Five. To the naked eye it looks like it is surrounded by dots in a diamond shape, under a magnifying glass those dots are zeros, fives and the word 'five' written over and over in tiny letters.
5. On the new £10 Jane Austen note, there are actually three women: the Queen, Jane Austen, and an illustration of Miss Elizabeth Bennet, the lead character in Pride and Prejudice.
6. The stately home on the £10 note isn't technically from any of Austen's novels - and wasn't somewhere she lived. It's Godmersham Park, her brother's house. She was a regular visitor, so people have speculated that it was the inspiration for a number of her novels.
7. On the Austen £10, there are also two images of Winchester Cathedral, where Austen is buried: the gold foil version on the front of the note is designed as a security feature.
8. There a number of other illustrations on the £10 note designed as security features - including a quill which changes colour from purple to orange, and a mini book-shaped copper foil patch, which contains the letters JA.
9. The quote on the Jane Austen note: "I declare after all there is no enjoyment like reading!" seems apt for the author, until you know a bit more about it. It's spoken by Miss Bingley in Pride and Prejudice, who has no interest in books at all, and is just trying to impress Mr Darcy.
10. The really clever stuff on the UK banknotes, meanwhile, is often hidden from view. One of these features is a set of circles on each note - often hidden in the design. The exact position of these circles acts as a coded message. Photocopiers by law have to be able to identify those codes, so that if someone tries to print a colour copy, instead they get a printout of a message on counterfeiting - in several languages. This has not been confirmed by the Bank of England, but was revealed in a BBC study a few years ago.
11. The same piece of work asserted that there was a hidden digital watermark on the notes too, invisible to humans but detected by editing programmes. This has also not been confirmed by the authorities, but would explain why editing software is unable to alter images of the banknotes.
12. The Euro banknotes, meanwhile, feature a number of bridges. These aren't real bridges at all - as it would be difficult to favour one in a particular country over a bridge elsewhere - so they are just stylised illustrations.
13. The bridges on Euro notes are supposed to symbolise communication between the people of Europe and the rest of the world.
14. Many of the Euro notes also feature open doorways, windows and archways. Again, these aren't real constructions, they are meant to symbolise the European spirit of openness and cooperation.
15. Perhaps the feature that will mean the most to people is on one single £5 note. Back in December artist Graham Short engraved five of the new Churchill £5 notes with a tiny picture of Jane Austen, plus a quote - making them worth tens of thousands of pounds. One was given to the Jane Austen Centre, and four were released into circulation. One of the notes is still out there, with the serial number AM 32 885554.