An animal door comprising: a frame adapted to be installed at a through-opening defined by a structure, said frame defining a passageway at the through-opening; a flap mechanically engaged with said frame, said flap being movable between a closed position and an open position; at least two metallic members disposed at either said frame or said flap; at least two magnets disposed at either said frame or said flap, said at least two magnets generating magnetic forces that draw at least two of said at least two magnets toward either another of said at least two magnets or at least two of said at least two metallic members, the magnetic force generating a resistance against moving said flap from the closed position; characterized in that at least one of said at least two metallic members and at least one of said at least two magnets are removably disposed proximate a perimeter of the flap aligned in pairs opposite each other to generate a magnetic force, wherein the pairs are arranged on the left side of the through-opening and on the right side of the through opening in mirror-image to a vertical axis, such that a magnitude of the collective magnetic force of the at least two magnets and the at least two metallic members creates an adjustable resistance against moving the flap from the closed position, the adjustable resistance having a first configuration corresponding to a maximum collective magnetic force and a second configuration corresponding to a reduced collective magnetic force relative to the maximum collective force based on the number of magnets and metallic members relatively disposed at the frame or flap, respectively. a frame adapted to be installed at a through-opening defined by a structure, said frame defining a passageway at the through-opening; a flap mechanically engaged with said frame, said flap being movable between a closed position and an open position; at least two metallic members disposed at either said frame or said flap; at least two magnets disposed at either said frame or said flap, said at least two magnets generating magnetic forces that draw at least two of said at least two magnets toward either another of said at least two magnets or at least two of said at least two metallic members, the magnetic force generating a resistance against moving said flap from the closed position; characterized in that at least one of said at least two metallic members and at least one of said at least two magnets are removably disposed proximate a perimeter of the flap aligned in pairs opposite each other to generate a magnetic force, wherein the pairs are arranged on the left side of the through-opening and on the right side of the through opening in mirror-image to a vertical axis, such that a magnitude of the collective magnetic force of the at least two magnets and the at least two metallic members creates an adjustable resistance against moving the flap from the closed position, the adjustable resistance having a first configuration corresponding to a maximum collective magnetic force and a second configuration corresponding to a reduced collective magnetic force relative to the maximum collective force based on the number of magnets and metallic members relatively disposed at the frame or flap, respectively.