Recent experiments on exotic nuclei with N=18 and 19 are discussed in connection to the variation of the shell gap at N=20. It is suggested that the gap is really small in such exotic nuclei. We show how the gap can be calculated from the shell‐model interaction SDPF‐M. Its characteristic feature can be explained as a effect of the tensor force. Thus, one of the subjects Dave Warner has been studying opens a new field of nuclear structure physics.