Auto selection in Search Form

People may place something like “Search this website…” in the search form instead of leaving it blank. But you have to place the cursor to the search form and delete all those words before you can input the keywords for search. Some website, however, have the search form designed in a way that when you click the form, the entire text “Search this website…” is auto selected. So you can press “DEL” right away without delete those letters one by one. How can I do that? Thank you!