Although multitasking with laptops and mobile phones has become endemic on university campuses, we still don't have adequate understanding of this phenomenon. There is especially a lack of qualitative studies offering detailed and rich description of how and why students multitask. This exploratory study provides a nuanced understanding of in-class multitasking with smartphones and laptops. The study involves sixteen students from a comprehensive university in Hong Kong in in-depth interviews. The results indicate that the university students mainly multitask with mobile phones on activities unrelated to the ongoing class. Lectures and peer presentation are reported as the activities students felt least engaged with. The study has revealed a new type of multitasking behavior, that is learning-related yet off-task. The multitasking behavior also fluctuates on an array of factors associated with motivation, classroom context, and the presence of technology.