1. A system for detecting a boiling event at a cooktop, the system comprising: a vision sensor positioned so as to collect imagery depicting the cooktop; one or more processors; and one or more non-transitory computer-readable media storing instructions that, when executed by the one or more processors, cause the one or more processors to perform operations, the operations comprising: obtaining a first frame of imagery and a second frame of imagery from the vision sensor, wherein the first frame of imagery and the second frame of imagery respectively depict the cooktop; identifying a plurality of motion vectors based on the first and second frames of imagery, wherein each of the plurality of motion vectors describes a change in position of one or more pixels from the first frame of imagery to the second frame of imagery; generating a histogram describing a characteristic of the plurality of motion vectors; calculating a dissimilarity score, wherein the dissimilarity score describes a difference between the histogram and a pre-learned histogram, and wherein the pre-learned histogram describes the characteristic for imagery depicting boiling liquid; and determining whether a boiling event is occurring at the cooktop based at least in part on the dissimilarity score.