Tell me how long it takes for light to travel and I will tell you how fast you go

The Italian Government's Ministry of Public Education has introduced the special theory of relativity for students in their last year of high school. For this reason, we often organize in-depth lessons on modern physics, in general, and on relativity theory in particular. Often, even simple new comers of physics participate. For example, we would like to show, in a subsequence lesson in the classroom, a relation proposed by the first author of this paper, a distinguished jurist and lover of general science. This relation allows us to deduce the Lorentz factor without knowing the relative velocity between the reference frames.