Snake protective case is adjustable. One size fits all glass cylinder pen cartridges. Silicone, ANTI-ROLL design stops pen from rolling and rubber coil protects glass from breaking. Special design will not block cartridge air flow.

Glow in the Dark cases need to be "charged" by placing in the sun or under a bright light. Glow in the Dark feature will last longer when the case is exposed to direct light for a longer period.