Surface similarity generally promotes reasoning by analogy and physical similarity has been shown to have a powerful positive effect on very young children's use of a scale model as a source of information about another space. The research reported here investigated 2 1/2-year-old children's performance in an object retrieval task when asked to reason from one space to a second, perceptually identical one. Contrary to the expectation that increased surface similarity would improve the children's performance, it actually had a negative effect. Children were less successful in 3 different tasks in which 2 spaces were identical than a task in which the spaces were merely similar in size and appearance. Their performance appeared to be affected by 4 factors: representational insight, memory for the most recent hiding place, the ability to resist responding perseveratively, and their level of analysis of the task. The results suggest that part of the power of symbols derives from the fact that they are virtually never identical to their referents, thereby making it possible to use one to draw inferences about the other without risk of confusing them. (Contains 4 figures and 4 footnotes.)