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Last Friday evening and Saturday day, we had a Healthy Hackathon at Khan Academy.

We “hacked” (coded) new ideas as fast as possible, but “healthily” (veggie snacks and no over-nighters). Then we voted on the projects.

One winner was intern Dylan Vassallo, who created a Chrome browser plugin for Wikipedia, “Khanpedia”. With it, when you visit Wikipedia and click on links there, you have the option of viewing the result in either Wikipedia or Khan Academy.

It is an unoffical hack project, but we think you’ll enjoy it!

Intern Ankit Ahuja and designer Kitt Hirasaki have launched the first of our planned summer improvements to the Khan Academy video discussions.

Now from every user’s profile page, you can view their history of questions, answers, and comments, as well as the discussion badges they’ve earned.

To view your own discussion history, visit your profile page and go to the “Discussion” tab.

Khan Academy has some amazing and involved contributors, and we hope to have many more! Check out some of their profiles:

So the next time you watch a video, ask or answer a question!

You may have heard of (or even celebrated) Pi day on 3/14, but how about Tau day? What is Tau, you ask? In 2001, Bob Palais published the article "π Is Wrong" in which he argued that the beloved constant π is the wrong choice of circle constant. He instead proposed using an alternate constant equal to 2π, or 6.283… to represent “1 turn”, so that 90 degrees is equal to “a quarter turn”, rather than the seemingly arbitrary “one-half π”.

Two years ago today, Michael Hartl published "The Tau Manifesto" echoing the good points made by Palais and building on them by calling this “1 turn” constant τ (tau), as an alternative to π. Tau is defined as the ratio of a circle’s circumference to its radius, not its diameter and is equal to 2π.

Tau is approximately equal to 6.28, so that makes today (6/28) tau day! Aside from eating twice as much pie as we would on 3/14, we’ve found another way to recognize tau day: Thanks to Emily Eisenberg, one of our awesome summer interns, all of our exercises that can be answered in terms of pi can now be answered in terms of tau too!

All the hints and explanations still use pi, and of course we still accept answers with pi, but for those of you in the know, you can now use this secret feature to answer with tau!

Try it out for yourself, and let us know what you think!

Using the app, you can view all 3,200+ videos available on the Khan Academy website, including all of Sal’s videos as well as art history videos by Beth Harris and Steven Zucker, math videos by Vi Hart, and cryptography videos by Brit Cruise.

For every video with English subtitles, the app includes an interactive transcript which allows you to read along with the video or go to a specific section if you want to rewind or jump ahead. If you log into your Khan Academy account, all of your progress is automatically saved so you can earn energy points and badges for the videos you watch.

Best of all, using the app, you can download videos so you can watch them anywhere without internet access, such as in an airplane or on a long car ride – you can either download a single video or all the videos in a single topic.

Currently, practice exercises are not available from within the iPad app but we hope to have them available in a future release; for now, you can do exercises directly on the website.

If you haven’t yet tried it, download it for free. We hope the app is a great tool for both students and teachers – enjoy!

Developer Ben Komalo has made it possible for parents to create accounts for their children who are under 13.

Parents, to create an account for your child, visit:

Child accounts have several safety features:

• The parent who creates the child account becomes the child’s permanent coach.
• The child cannot enter certain information (e.g.. email, full name) in their profile, nor make their profile public.
• The parent has the option of disabling the child from adding other people as coaches. (See What is a coach? for more details on coaching.)
• The parent manages the child’s password and can change it at any time for any reason.
• The child cannot post public messages in discussion forums.