A pen includes a pen tube, a liquid guide accommodated in the pen tube, a tail sleeve detachably connected to the pen tube, and a liquid cartridge. The liquid cartridge is accommodated in the tail sleeve and extends into the pen tube. The liquid cartridge includes a container and a plug structure. The container has an opening. The plug structure is disposed in the container and includes a cover portion, an elastic connection portion, and an annular wall portion. The elastic connection portion is connected to the cover portion and has a hollow portion. The annular wall portion is connected to the elastic connection portion and surrounds the cover portion. An annular gap is formed between the cover portion and the annular wall portion and communicated with the hollow portion. The liquid guide is pressed against the cover portion to open the annular gap.