Play Tube is a free app that let's you download YouTube videos so you can play them later, even if you aren't connected to the internet. I've used it a couple times and it hasn't failed me yet. I even have a few workout videos downloaded so I can sweat it out no matter where I am. However, the downside of this app is that the videos do take up your gigabytes so you'll have to be careful with the number of videos you choose to download and perhaps even delete them when you are finished using them. That part might get a bit tedious, but it's better than planning to use a YouTube video to introduce or explain a complex topic and then realizing that the school's internet is down. Now if the power goes out, you are on your own! (Or you can have your 20-some students huddle around the iPad to watch.) Either way, I hope you find Play Tube to be a useful app.