A system comprising: a transmitter ; a fluid-sensitive battery comprising: a first electrode , wherein the first electrode comprises magnesium metal; a second electrode , wherein the second electrode comprises an absorbent conductive material; and a separator , wherein the separator is in contact with both the first electrode and the second electrode , wherein the separator comprises an absorbent material, and wherein a salt is disposed within at least one of the separator or the absorbent conductive material of the second electrode , wherein the salt comprises at least one of sodium chloride and potassium chloride, wherein the battery provides an electrical voltage between the first electrode and the second electrode when exposed to an aqueous fluid; and a controller operably coupled to the transmitter and the fluid-sensitive battery , wherein the controller operates the transmitter to transmit a radio-frequency signal in response to the battery providing the electrical voltage, wherein the transmitter and the controller are incorporated into a diaper, wherein the diaper comprises first and second conductive clasps that can be connected to each other and disconnected from each other; wherein the controller is operably coupled to the fluid-sensitive battery via the first and second conductive clasps, and wherein the conductive clasps are arranged to determine whether the diaper is being worn by a wearer and/or arranged to determine that the diaper has been removed. a transmitter ; a fluid-sensitive battery comprising: a first electrode , wherein the first electrode comprises magnesium metal; a second electrode , wherein the second electrode comprises an absorbent conductive material; and a separator , wherein the separator is in contact with both the first electrode and the second electrode , wherein the separator comprises an absorbent material, and wherein a salt is disposed within at least one of the separator or the absorbent conductive material of the second electrode , wherein the salt comprises at least one of sodium chloride and potassium chloride, wherein the battery provides an electrical voltage between the first electrode and the second electrode when exposed to an aqueous fluid; and a first electrode , wherein the first electrode comprises magnesium metal; a second electrode , wherein the second electrode comprises an absorbent conductive material; and a separator , wherein the separator is in contact with both the first electrode and the second electrode , wherein the separator comprises an absorbent material, and wherein a salt is disposed within at least one of the separator or the absorbent conductive material of the second electrode , wherein the salt comprises at least one of sodium chloride and potassium chloride, wherein the battery provides an electrical voltage between the first electrode and the second electrode when exposed to an aqueous fluid; and a controller operably coupled to the transmitter and the fluid-sensitive battery , wherein the controller operates the transmitter to transmit a radio-frequency signal in response to the battery providing the electrical voltage, wherein the transmitter and the controller are incorporated into a diaper, wherein the diaper comprises first and second conductive clasps that can be connected to each other and disconnected from each other; wherein the controller is operably coupled to the fluid-sensitive battery via the first and second conductive clasps, and wherein the conductive clasps are arranged to determine whether the diaper is being worn by a wearer and/or arranged to determine that the diaper has been removed. <EOS>