Tau-theta paradox (1950s): Whenever two distinct kinds of kaons, tau and theta (nowadays tau refers to a totally different particle) decay, tau decays into three particles, whereas the theta decays into two. The tau and theta vary only in parity; and at the time, it was considered that parity was firmly conserved and that particles varying only in parity must behave exactly similar. As the two decay differently, a paradox developed. The paradox was solved whenever experiments taken out according to the F. Yang and T.D. Lee's theoretical computations indeed point out that parity is not conserved in the weak interactions