Background: Children are not blank slates when they begin school; they bring prior conceptions about the everyday world with them. These conceptions usually do not comply with accepted scientific views and have to be changed within the process of education. However, to do this effectively more needs to be known about the relationship between the everyday world and children's knowledge of scientific principles. Purpose: This study sought answers to the question of which object variables children use when reasoning, and how these variables are associated with outcomes. The reported study addresses these issues in relation to object motion. Sample, design and methods: UK primary school children (n = 144) aged 5-11 years were assessed on their predictions of motion along a horizontal, in fall and down an incline using a range of everyday objects by responding to questions where they needed to compare potential motion patterns of the objects. Results: Round shape and smooth texture of objects were consistently associated with faster motion across age groups as well as across motion dimensions. However, faster horizontal motion was associated with lighter and smaller objects across all ages, whereas faster fall was associated with heavier objects. While younger children predicted faster incline motion for lighter and smaller objects, there was a shift in conceptions with age, with older children predicting faster motion for heavier and bigger objects. Conclusions: The overall findings are used to support the development of commonsense theories of motion previously identified, and suggestions for educational practice are made. Specifically, it is suggested that these findings may need to be taken into consideration in the development of teacher training programmes. (Contains 3 figures.)