This exploratory study in a classroom setting investigates first graders' (age 7-8 years, N = 25) ability to perform analogical reasoning and create their own analogies for two irreversible natural phenomena: mixing and heat transfer. We found that the children who contributed actively to a full-class discussion were consistently successful at making analogical comparisons between known objects provided by a researcher and that some of the children could come up with their own analogies for the abstract natural phenomena with which they interacted. The use of full-class and small-group settings, shared laboratory experiences of the phenomena and children's drawings as different kinds of scaffolding was found to be helpful for the children's analogical reasoning. As an implication for science education, self-generated analogies are put forward as a potential learning tool within a constructivist approach to education.