The solution to the Euler-Lagrange equation is an extremal functional. To understand that the functional is stationary at local extrema (maxima or minima), we propose a physics experiment that involves using a soap film to form a catenoid. A catenoid is a surface that is formed between two coaxial circular rings and is classified mathematically as a minimal surface. Using the soap film, we create catenoids between two rings and characterize the catenoid "in situ" while varying the distance between the rings. The shape of the soap film is very interesting and can be explained using dynamic mechanics. By observing the catenoid, physics students can observe local extrema phenomena. We stress that "in situ" observation of soap-film catenoids is an appropriate physics experiment that combines theory and experimentation. (Contains 9 figures.)