Steaks earn you lingots, which are Duolingo currency.

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What is a lingot?

A lingot [ling-guht] is the Duolingo virtual currency. The more you learn on Duolingo, the more lingots you'll receive and be able to use in the store! They can be earned through:

I.e. 10 day streaks Earn 1 lingot for every 10 days on a streak (1 for 10, 2 for 20, etc).

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You can exchange them for stuff in the store to help you while you practice. http://www.duolingo.com/show_store