We approached the first and second challenges by using a Bayesian approach to learn which terms were associated with events, regardless of whether they are standard language, acronyms, or even a made-up word, so long as they match the events of interest. The third and fourth challenges are approached by using word-pairs, where we extract all the pairs of co-occurring words within each tweet. To find the words most associated with events, we search for the words that achieve the highest number of spikes matching the days of events.  
 Question: How are the keywords associated with events such as protests selected?
By using a Bayesian approach  and by using word-pairs, where they extract all the pairs of co-occurring words within each tweet.  They search for the words that achieve the highest number of spikes matching the days of events.