In recent years, social bots have been using increasingly more sophisticated, challenging detection strategies. While many approaches and features have been proposed, social bots evade detection and interact much like humans making it difficult to distinguish real human accounts from bot accounts. For detection systems, various features under the broader categories of account profile, tweet content, network and temporal pattern have been utilised. The use of tweet content features is limited to analysis of basic terms such as URLs, hashtags, name entities and sentiment. Given a set of tweet contents with no obvious pattern can we distinguish contents produced by social bots from that of humans? We aim to answer this question by analysing the lexical richness of tweets produced by the respective accounts using large collections of different datasets. Our results show a clear margin between the two classes in lexical diversity, lexical sophistication and distribution of emoticons. We found that the proposed lexical features significantly improve the performance of classifying both account types. These features are useful for training a standard machine learning classifier for effective detection of social bot accounts. A new dataset is made freely available for further exploration.