See also: The Forum
This is the sandbox, a page for conducting large editing experiments. To use the sandbox, click on edit at the top of the page, post your experiment below the line, and click Save page when you are finished. Note that this is a temporary page, so content added here will not remain permanently.
For smaller or repetitive experiments, simply use the Show Preview button before saving your experiments to the page you are seeking to modify (this prevents abuse of the sandbox by avoiding inappropriate flooding of the recent changes page).