Collective excitations are an emergent phenomena. They are can look unlike the constituent particles of the system, but they depend on--and can probe--the underlying symmetries and many-body physics. In this talk I will focus on magnons, or spin wave excitations. We demonstrate a precise measurement of the magnon dispersion relation in the ferromagnetic phase and observe a many-body gap due to dipolar interactions. I will discuss applications of magnons towards thermometry and cooling of a Bose-Einstein condensate.