The newly revised mathematics curriculum in Singapore has recently factored Applications and Modelling to be part of the teaching and learning of mathematics. Its implication is that even children should now be involved in works of mathematical modelling. However, to be able to implement modelling activities in the primary mathematics classroom, it is at the outset needful for teachers to have an understanding of what it is, how it is different from conventional pedagogies, and what the learning outcomes look like. Since the goal of the Singapore mathematics curriculum is problem solving, I discuss mathematical modelling as problem solving and examine the mathematical reasoning of two groups of high-ability Primary 6 students in their engagement of a model-eliciting task based on a classroom research. Comparisons of the two groups within the modelling stages show that primary school students were able to reason mathematically towards constructing models. (Contains 3 figures.)