In this lecture from the second year 'Fluid Dynamics and Waves' course, Dominic Vella discusses what a fluid is, how it differs from a solid, and how to describe the kinematics of its motion. He focuses on two different reference frames (the laboratory frame and a frame that moves with the fluid) and discusses how to relate rates of change of quantities between these two reference frames. You can watch many other student lectures via our main Student Lectures playlist (also check out specific student lectures playlists):    • Oxford Mathematics Student Lectures -...   All first and second year lectures are followed by tutorials where students meet their tutor in pairs to go through the lecture and associated problem sheet and to talk and think more about the maths. Third and fourth year lectures are followed by classes.