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Differences in cellular phones' complexity and their impact on children's performance are under study in this experiment. Twenty children (age 9-14 years) solved tasks on two phones that were simulated according to existing models on a PC with a touch screen, holding constant display size, fonts, and colors. Actions were logged and analyzed regarding execution time, detour steps, and specific errors. Results show that children using the Siemens C35i with 25% higher complexity (with regard to number of required production rules) spent double the time solving tasks and undertook three times as many detour steps as children using the less complex Nokia 3210. A detailed analysis of user actions revealed that the number of production rules to be learned fails to account for most difficulties. Instead, ambiguous naming, poor categorization of functions, and unclear functionality of keys undermined performance. Actual or potential applications of this research include guidelines to improve the usability of all devices with small displays and hierarchical menu structures, such as cellular phones. [PUBLICATION ABSTRACT]