Teachers beware!

It's a misconception that

• All web-pages are available indefinitely, without any time limit. This is not always the case. You should check the web site addresses before hand, to see whether they had limited life or still alive, before suggesting them to your students.
• The contents of a webpage do not change. This may put you in trouble, especially links to undesirable web sites are added, after your last visit to the site. Check the page, just before the lesson, and keep watching for a reasonable period of time, as the students may be accessing the page on your advise.