Answer:
I drift forever with the current down these long canals theyβve made, Tame, yet wild, I run elusive Multitasking to your aid. Before I came, the world was darker, colder, sometimes, rougher, true. But though I might make living easy, Iβm good at killing people too.
My first master has four legs, my second master has two. My first I serve in life, my second I serve in death. Tough I am, yet soft beside. Against ladies cheeks I often reside.
It comes only before, it comes only after. Rises only in darkness, but rises only in light. It is always the same, but is yet always different.
Metal or bone I may be, many teeth I have and always bared. Yet my bite harms no one. And ladies delight in my touch.
Tall in the morning, short at noon, gone at night. But I'll be back soon.
No matter how little or how much you use me, you change me every month.