In three experiments, children's reliance on other people's testimony as compared to their own, first-hand experience was assessed in the domain of ontology. Children ranging from 4 to 8 years were asked to judge whether five different types of entity exist: real entities (e.g. cats, trees) whose existence is evident to everyone; scientific entities (e.g. germs, oxygen) that are normally invisible but whose existence is generally presupposed in everyday discourse; endorsed beings (e.g. the Tooth Fairy, Santa Claus) whose existence is typically endorsed in discourse with young children; equivocal beings (e.g. monsters, witches) whose existence is not typically endorsed in discourse with young children; and impossible entities (e.g. flying pigs, barking cats) that nobody believes in. Children make a broad dichotomy between entities and beings that they claim to exist (real entities; scientific entities; and endorsed beings) and those whose existence they deny (equivocal beings and impossible entities). They also make a more fine-grained distinction among the invisible entities that they claim to exist. Thus, they assert the existence of scientific entities such as germs with more confidence than that of endorsed beings such as Santa Claus. The findings confirm that children's ontological claims extend beyond their first-hand encounters with instances of a given category. Children readily believe in entities that they cannot see for themselves but have been told about. Their confidence in the existence of those entities appears to vary with the pattern of testimony that they receive.