What are some interesting exercises to learn Hadoop?

What are some interesting exercises to learn Hadoop?
1) Start with basic word count.
2) Process Wikipedia articles to get some interesting facts.
3) Process Twitter logs e.g - sentiment analysis, trending topics, trending stocks, market feeling about certain product etc.
4) Use weather dataset and try to device modeling algorithm for predicting future weather.
5) Build your own search index
on any large corpus (It was one of the assignment for us at Stanford  for Information Retrieval and Web Search class ).
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