GoAnimate is a site that lets you easily create animated videos. There is a free version and a discounted education version. The free version allows you to easily make the videos, record your own voice or use the built in ones, and share the video. The Schools account is a private environment, with school safe content and moderation available and includes all of the premium features at a discounted rate.
GoAnimate can be used for teacher presentations, student projects, tutorials, ELL/ESL training, and much more. Students can have fun creating a presentation. They will learn the content, as well as how to organize a presentation and follow certain guidelines. There are great help tools available and the character creator makes it easy to create the characters in your video.
Here's an example of one on Diffusion and Osmosis
GoAnimate4Schools.com: Diffusion & Osmosis by Paul Tan
Like it? Create your own at GoAnimate4Schools. It's free and fun!
You can also browse for already created videos to use with your classes.