  We present a simple activity based on the liquid-drop model which allows
secondary school students to explore the uses of mathematical models and gain
an intuitive understanding of the concept of binding energy, and in particular
the significance of positive binding energy. Using spreadsheets provided as
Supplementary Material, students can perform simple manipulations on the
different coefficients of the model to understand the role of each of its five
terms. Students can use the spreadsheets to determine model parameters by
optimising the agreement with real atomic mass data. %This will subsequently be
used to predict the limit of existence of the Segr\'e chart and to find the
minimum mass of a neutron star. This activity can be used as the starting point
of a discussion about theoretical models, their validation when it comes to
describing experimental data and their predictive power towards unexplored
regimes.
