Two studies were conducted to understand why subtraction with fluency is harder than addition. In Study I, 33 kindergartners were individually asked to transfer cubes from a glass to an empty bottle, one by one, with one-to-one correspondence with the interviewer. They were then asked if the quantities remaining in the two glasses were the same and if the quantities in the two bottles were the same. In Study II, 21 first-graders and 38 fourth-graders were asked mental-arithmetic questions such as 8+2 and corresponding subtraction questions such as 10-8. By analyzing children's accuracy and reaction time, it was concluded, in light of Piaget's theory, that subtraction is harder than addition because children deduce differences from their knowledge of sums. The educational implication is that we need to deemphasize subtraction in the primary grades and make sure that children's knowledge of sums is solid. © 2001 Elsevier Science Inc.