Two-neutron halo nuclei break into three fragments and are called Borromean because of this behavior, analogously to how all three of the Borromean rings are linked together but no two share a link. For example, the two-neutron halo nucleus He (which can be taken as a three-body system consisting of an alpha particle and two neutrons) is bound, but neither He nor the dineutron is. He and Be both exhibit a four-neutron halo.