I love tongue twisters! Although the words in them are usually easy to understand, they totally mess up your head! It's simple fun to get out of a bored state of mind or to freshen up sleepiness. Plus, it's good for fluency practice as well! If it doesn't, at least the students and you will be amused :)
Here's a site I found that's pretty cool for this, check it out:
For English, AskOxford says, nobody can really decide.
"Most of the words which are given as 'the longest word' are merely inventions, and when they occur it is almost always as examples of long words, rather than as genuine examples of use..."
Quite an interesting (short) article :)
[ link : @AskOxford.com (sub of Oxford University Press) ]
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For any language in general, however, there are many words which are loooong.
Here's a blog about teaching languages using, mostly, interactive games. His ideas are mainly technology-based (eg. using the computers, internet, etc), and/but they're very entertaining. They can be used for days when you feel like stepping away from being at the front of the class, and letting the kids have fun (while secretly learning ;) ).