An early device developed for concealing intelligence reports when traveling by water was a simple weighted bottle that could be dropped overboard if there was a threat of capture. This was replaced by a wafer-thin leaden container in which a message was sealed. It would sink in water, and melt in fire, and could be used by agents on land or water. It had one drawback—lead poisoning if it was swallowed. It was replaced by a silver, bullet-shaped container that could be unscrewed to hold a message and which would not poison a courier who might be forced to swallow it.