The authors discuss mass as one of the three fundamental measurements (the others being length and time), noting that estimation of mass is little taught and assessed in primary schools. This article briefly explores the reasons for this in terms of culture, practice, and the difficulty of assessing estimation of mass. An activity using the differences between a set of nested eggs is outlined. Use of this activity may help to promote the exploration of mass estimation concepts as well as the assessment of mass in a classroom environment. Students' general misconceptions in relation to mass, their limited formal and informal experiences with estimating mass, as well as the infrequent assessment of estimation of mass concepts are highlighted as springboards for this study. (Contains 6 figures.)