A closure (20, 20A, 20B) for a container, said container having an opening to the interior of the container wherein contents may be stored and an outwardly projecting flange, said closure (20, 20A, 20B) comprising: a base (40, 40A, 40B) for being installed on the container at the container opening, said base (40, 40A, 40B) having an access passage through said base (40, 40A, 40B) for permitting access to the container interior; and a closing element (42, 42A) operable on said base (40, 40A, 40B) between a closed position for occluding said access passage and an open position for permitting communication with said access passage, and wherein said base (40, 40A, 40B) has a skirt (50, 50A, 50B) for attaching said closure (20, 20A, 20B) to the container, said skirt (50, 50A, 50B) having (i) an exterior surface (52, 52A, 52B), (ii) an interior surface (53, 53A, 53B), (iii) a lower end portion (54, 54A, 54B) for being located laterally outwardly of the container flange when said base (40, 40A, 40B) is installed at the container opening, said lower end portion (54, 54A, 54B) defining an annular, lower end surface (122, 122A, 122B), and (iv) an annular, flexible retention member (58, 58A, 58B) extending laterally inwardly from said lower end portion (54, 54A, 54B) so as to define a connection (62, 62A, 62B) therebetween, said flexible retention member (58, 58A, 58B) having (a) an initial, undeflected configuration, (b) a deflected configuration rotated away from said initial, undeflected configuration about said connection (62, 62A, 62B), characterized in that said flexible retention member (58, 58A, 58B) further comprises (c) a proximal end surface (126, 126A) located at said connection (62, 62A, 62B), said proximal end surface (126, 126A) and said lower end surface (122, 122A, 122B) together defining a shoulder when said flexible retention member (58, 58A, 58B) is in said undeflected configuration, wherein said proximal end surface (126, 126A) of said flexible retention member (58, 58A, 58B) in said undeflected configuration is normal to said lower end surface (122, 122A, 122B) when viewed in a vertical cross-sectional plane taken through a central axis of said closure (20, 20A, 20B). a base (40, 40A, 40B) for being installed on the container at the container opening, said base (40, 40A, 40B) having an access passage through said base (40, 40A, 40B) for permitting access to the container interior; and a closing element (42, 42A) operable on said base (40, 40A, 40B) between a closed position for occluding said access passage and an open position for permitting communication with said access passage, and wherein said base (40, 40A, 40B) has a skirt (50, 50A, 50B) for attaching said closure (20, 20A, 20B) to the container, said skirt (50, 50A, 50B) having (i) an exterior surface (52, 52A, 52B), (ii) an interior surface (53, 53A, 53B), (iii) a lower end portion (54, 54A, 54B) for being located laterally outwardly of the container flange when said base (40, 40A, 40B) is installed at the container opening, said lower end portion (54, 54A, 54B) defining an annular, lower end surface (122, 122A, 122B), and (iv) an annular, flexible retention member (58, 58A, 58B) extending laterally inwardly from said lower end portion (54, 54A, 54B) so as to define a connection (62, 62A, 62B) therebetween, said flexible retention member (58, 58A, 58B) having (a) an initial, undeflected configuration, (b) a deflected configuration rotated away from said initial, undeflected configuration about said connection (62, 62A, 62B), (a) an initial, undeflected configuration, (b) a deflected configuration rotated away from said initial, undeflected configuration about said connection (62, 62A, 62B), characterized in that said flexible retention member (58, 58A, 58B) further comprises (c) a proximal end surface (126, 126A) located at said connection (62, 62A, 62B), said proximal end surface (126, 126A) and said lower end surface (122, 122A, 122B) together defining a shoulder when said flexible retention member (58, 58A, 58B) is in said undeflected configuration, wherein said proximal end surface (126, 126A) of said flexible retention member (58, 58A, 58B) in said undeflected configuration is normal to said lower end surface (122, 122A, 122B) when viewed in a vertical cross-sectional plane taken through a central axis of said closure (20, 20A, 20B).