Smartphones, and other similar devices, are ideal for private activities: They are carried on the body and can be physically secured from prying eyes. They work anywhere. And they are not typically shared among family members or classmates, making them perfect for searching and exploring sensitive information - both via web search or through social information seeking. This paper validates these intuitions empirically, and builds on these observations to investigate the expression of sensitive information needs, such as inquiring about unplanned pregnancies, among users of mobile devices. We examine a large set of queries from the United States (4.2 million), submitted to Yahoo! Answers, and compare these to the 6.7 million queries submitted to Yahoo! search. Focusing on community question answering allows much richer analysis of the way information needs are expressed, setting our work apart form previous studies. For the first time, we empirically show that people prefer to express sensitive needs on mobile devices, as this manifests at both the lexical level and the semantic level, using a pre-defined taxonomy of topics. Further, we find that preference for mobile devices for sensitive topics holds true even when controlling for age and gender, which is facilitated by the large sample of users in our study (1.5 million). In particular, we show that young users, especially females, are more likely to inquire on sensitive issues, and are more likely to do this from locations distant from their common place of web access. To our knowledge, this is the largest study to date of mobile social information seeking, and is unique in terms of demographic diversity, the granularity of information needs examined, and the analysis of the location-dependence of sensitive information needs.