This article reports on work undertaken by schools as part of Qualifications and Curriculum Authority's (QCA's) "Engaging mathematics for all learners" project. The goal was to use in the classroom, materials and approaches from a Royal Institution (Ri) Year 10 master-class, "Number Sense", which was inspired by examples from Michael Blastland and Andrew Dilnot's (2007) book, "The Tiger That Isn't: Seeing through a world of numbers". Underpinning the master-class is a notion that to be numerate means much more than to be proficient with arithmetic calculations and algebraic manipulations. It means to have a true sense of what the particular numbers of interest stand for, their magnitude and import being determined by their context, whether they be percentages, rates, averages, measurements, forecasts, rankings, targets, ...; it means not being confused by a clutch of obfuscating figures, being able to efficiently extract a meaningful estimate, and communicate this to others. Depending on the complexity of the chosen examples, elements of the "Number Sense" master-class can be used with a wide range of secondary students across KS3 and KS4.