Comparing expert and novice performance is fruitful approach in research on mathematical problem solving, but is there something like expertise in children aged 10-12? The problem solving processes of 10 "pupil experts" ( successful participant of mathematical competitions) are compared to those of 45 " novices" (regular pupils). The "experts" show superior performance in all of the three tasks that were chosen for this study as well as higher " mental flexibility" in executing the processes which seems to distinguish them from the novices beyond advance in practice.