1: The chemical element phosphorous has the atomic number 15

In its pure form, phosphorous is highly reactive, but it’s not actually found in this form on Earth. Take a look at the chemical elements and how they are arranged in the Periodic Table in our free course, Discovering Chemistry.

2: Our 15th most visited free course on OpenLearn this year was…

Introduction to cyber security: stay safe online. The course will help you to understand online security and start to protect your digital life, whether at home or work. You will learn how to recognise the threats that could harm you online and the steps you can take to reduce the chances that they will happen to you.

3: Andy Warhol once said “In the future, everybody will be world-famous for fifteen minutes”

And with social media platforms such as TikTok and Instagram, the Pop Artist prediction was prophetic! But how can you navigate the world of social media, filter bubbles and fake news? This animation series, Social media - fake news, filter bubbles and sharing wisely, explores.

4: In Shakespeare’s Julius Caesar, Caesar is warned to beware the Ides (15th) of March

On a related note, could you get away with rewriting Shakespeare’s best work for the modern age? Grab your quill and take the test. Discover your inner writer in this short word game, Fakespeare, that matches you with one of Shakespeare's alleged co-authors.

5: In Rugby Union, a game is played between two teams of 15 players each

With such big teams, communication is key. In this free course, Communication and working relationships in sport and fitness, you will boost your ability to vary your communication approach according to the situation and the needs of the people involved.

6: Henry Addington was the 15th Prime Minister of the UK from 1801 to 1804

But how do you get the top job? This short video, How do you become British Prime Minister?, has some advice on the mechanics of the political system.

7: A fifteenth wedding anniversary is called a crystal anniversary

But is a crystal a rock? Crystals, rocks and minerals are three different materials. You can find out more in our Geology Toolkit, which includes a geology timeline, rock analyser, rock cycle, landscape features and safety tips.

8: Fifteen to One had a reputation for being one of the toughest quizzes on TV

We don’t have any quizzes quite as tough on OpenLearn but you can try our Photographic Memory test to see whether you have eidetic memory (that’s being able to vividly retain an image in your mind after only brief exposure).

9: On a Scrabble board there is a grid of 15×15 squares

Want to win this popular word game? Improve your current English skills or perhaps remember areas you may have forgotten in our free course, Everyday English.

10: Our 15th most visited interactive on OpenLearn this year was…

No sex please, we're British! There have been major changes in sexual attitudes over the years, but what are the views of the ‘average’ British person in terms of sex? This sexual attitudes interactive explores how your views compare to the majority.

11: In mathematics the number 15 is a lucky number, a triangular number, a hexagonal number and a pentatope number

If maths feels like a bit of a mystery to you and you’re looking to improve your confidence in using maths in a variety of everyday situations, our free course, Succeed with maths – Part 1, is for you.

12: Elizabeth I was crowned Queen of England on 15 January 1559 in Westminster Abbey, London

Does the reality of the life of Queen Elizabeth I match up with her reputation? Find out in our interactive, Behind the Reputation.

13: Our 15th badged course to be released on OpenLearn was…

Developing Career Resilience. This free badged course will help you to understand the factors that influence career resilience, and offers examples and tactics for you to develop yours further.

14: In the UK, people aged 15 can be sent to prison to await trial

Dr David Scott takes a look at the consequences of imprisoning children and young people, and the alternative solutions to this, in this mini documentary with accompanying article, Why we should abolish imprisonment for children and young people.

15: In Hebrew, the number 15 is spelled differently

This is because the two words, which usually make up fifteen, ten (yodh) and five (heh), when combined, make up one of the Jewish words for God! To avoid this, the number fifteen uses the words for nine (teth) and six (vav). Explore the variety and vibrancy of religion in contemporary Britain in our free course, Religious diversity: rethinking religion.