We love this pen - it is a really simple idea and one that really should be more widely available. The pen itself is a simple, clear-barrelled rollerball pen, and the secret is that it takes ink cartridges. Any universal ink cartridge will fit, and the real beauty is that Herbin themselves make small universal cartridges in a range of colours and these come in small metal tins. Put the pen and a selection of ink tins together and you have an a wonderful little set - get a pen and two tins for under £10 (at the time of writing!).