Zener cards are a set of cards used in experiments to test psychic abilities, particularly extrasensory perception (ESP). Each card features one of five symbols: a circle, cross, waves, square, or star. The cards were developed in the 1930s by psychologist Karl Zener and are often used in parapsychology research. In a typical experiment, one person, the "sender," looks at a card while another person, the "receiver," tries to guess the symbol. The goal is to determine if the receiver can accurately identify the symbols more often than chance would allow, providing insight into the potential for psychic phenomena.