To use demonstrations of those wonderful applets such as Building Houses that work on modern browsers, go to the Numworx Secondary Education Site. To use the resources without saving any work you can simply Login as Guest. Whilst a guest login has restrictions you will be able to explore and use resources, many of which make excellent demonstrations.
A site very well worth exploring.
The Cube buildings exercises include 4 sets of 10 exercises to keep everybody busy! The last set of 10 involves building structures given only the silhouette – quite a challenge. It takes a little bit of pratise to become familiar with the mouse controls, a left click adds a cube. To delete a cube hold the left click button and press the right click button also.
Further examples of these applets are on the page, Wisweb Applets HTML5.