This pen has a retractable tip, which makes it very convenient to use.

retractable 是由动词 retract（缩回、收回）加上后缀 -able（能够...的）构成的形容词。retract 本身由前缀 re-（表示"回"）和 tract（拉、拖）组成。retractable 意思是"能够缩回的"或"可伸缩的"。