To use much of the interactive content on this site, you need to have something called Java installed on your machine. Java is a programming language that is used (among other things) to write software that runs in web browsers. Java is free to download and is available for many different systems, including Windows, Mac OS and Linux.
You need a fairly recent version of Java to run much of the content. It is possible that you already have Java, but that the version you have is out of date.
Depending on your web browser, you may also need to tell it to allow the interactive content to run.
If you bought your computer recently, there's a good chance it already came with Java installed.
You can check if you have Java, and if it's an up-to-date version, at the following address. From this address you can also download the most up-to-date version of Java:
To check if you have Java (and what version you have), see the Do I have Java? link. If you know that you don't have it and need to install it, you can click the Free Java download button directly.
It may be that you have Java installed, but your browser is blocking it. Look out for a message such as the following at the top of your browser window:
If you see this message, then click on it and a small menu will appear. From this menu, select Allow blocked content. You will probably then need to reload the page. You can usually reload the page by pressing F5 or Cntrl + R, or by clicking the icon with two arrows inside it that appears next to the web address at the top of your browser.
Unlike some other forms of interactive content, Java is designed to be reasonably secure. Java programs running in web browsers are generally not permitted to perform potentially insecure actions, such as accessing your printer or hard disk, without your permission. So the risk from running Java programs in web pages, such as the ones on this web site, is minimal.
(Note that this security protection generally applies to Java programs running in web pages. If you download a Java application and run it outside your browser, you run most of the risks that you incur when running any program on your computer.)