Mobile HCI studies are often conducted in a highly controlled environment and with a small convenient sample. Such common studies can have a high internal validity but often lack external validity. The findings cannot always be generalized to the behaviour of real users in real contexts. In contrast, researchers recently started to use apps as an apparatus for mobile HCI research. Publishing apps in mobile application stores enables to study large samples in their 'natural habitat'. We present the game Hit It! that has been installed 427,255 times and, more importantly, served as a tool to conduct a number of HCI studies. The amount of collected data enabled to study users' touch behaviour, approaches to ask for consent, the best time to deploy games, and approaches for providing tactile feedback. We discuss advantages of the approach and argue that Hit It! enabled findings that would be impossible to obtain using traditional studies.