The purposes of this qualitative study were (a) to investigate the factors that support the generation of spontaneous analogies (SAs) by students, and (b) to investigate the factors that interfere with comprehension of the analogy target of an SA. To promote the generation of SAs, eight algebra-based physics students were asked to participate in two group problem-solving sessions and one individual explaining session. Overall, 18 spontaneous analogies were generated. Two factors appeared to support the spontaneous generation of analogies: (1) sufficient prior understanding of the analogy target, and (2) the existence of previous analogical examples from other students. Two factors appeared to interfere with the comprehension of the SA analogy target: (1) an incorrect perception of the type of analogy between target and base (attributes, relationships), and (2) an incorrect understanding of the analogy base.