<human>:  With regards to superconductivity and Josephson junctions, how do you know that a superconductor is superconducting?
<bot>: You can test for superconductivity using a Josephson junction. In a Josephson junction, two superconductors are separated by a thin insulating barrier. If the barrier is thin enough, electrons can tunnel through the barrier and travel across the junction without resistance. This is called the Josephson effect. If the barrier is too thick, the electrons will not be able to tunnel through the barrier, and the junction will behave as an ordinary conductor.