This document discusses sentiment analysis techniques for classifying tweets based on their positive, negative, or neutral sentiment. It proposes two Latent Dirichlet Allocation (LDA) based models - Foreground and Background LDA (FB-LDA) and Reason Candidate and Background LDA (RCB-LDA) - to analyze sentiment variation in tweets. FB-LDA can filter background topics and extract foreground topics to identify possible explanations for sentiment changes. RCB-LDA can rank reason candidates expressed in tweets to provide sentence-level sentiment explanations. The proposed techniques are intended to classify tweets and evaluate public sentiment variations by extracting possible reasons for those variations.