The Internet is a great resource for teaching. There are thousands, more, millions of great videos, tutorials and animations to use in classes. But the problem with the internet is that nothing stays the same, pages come and go, problems happen, schools block youtube and then you lose that great 5 minute video explaining photosynthesis.
So save them, copy them, download them and then share them. But some sites do not allow you to copy them. Here’s how, use firefox and add these two neat add ons.
https://addons.mozilla.org/en-US/firefox/addon/201 for animations
If you do not use firefox, then http://www.saveyoutube.com is often a good substitute.