Measuring the pressure inside a party balloon with a ruler

We measure the radius of a party balloon after consecutive exhalations. We use the stress–strain relation for latex, and argue that the inflation is consistent with a constant volume of air exhaled in a single breath. We find its value and determine the gauge pressure of the balloon by applying the principle of least work. We compare elastic properties of party balloons with those of the aorta. The experiment illustrates methods from metrology in a context suitable for the introductory sequence.