Embodiments of a plug and cap of a Universal-Serial-Bus (USB) device have been presented. In one embodiment, a USB device includes a main body, a piece of string, and a cap. The main body has a printed circuit board assembly (PCBA) and a casing, wherein the PCBA is partially housed in the casing, and the PCBA further includes a USB connector protruding out of the casing at a first end of the casing. The piece of string is coupled to the main body and the cap. The cap is detachably coupled to the first end of the casing of the main body to cover the USB connector, wherein the cap remains indirectly coupled to the casing via the piece of string when the cap is detached from the first end of the casing to expose the USB connector.