If you're looking for an engaging explanation of a scientific concept, here are a couple of sites that offer videos for use in the classroom.
Science Mag - aimed at older (secondary) learners and each free video has an accompanying text (which unfortunately is only available via subscription). Videos cannot be embedded, only linked to but this site's well worth a visit. The accompanying overview of the articles could be manipulated for various tasks such as gap-fills, text ordering, true/false exercises etc.
Simple Science - possibly better for younger learners or at least those with a sense of humour! The songs can be memorised and practised among groups of learners or used to listen for particular facts.