Answer:
I open wide and tight I shut, Sharp am I and paper-cut fingers too, so do take care, I'm good and bad, so best beware.
Different lights do make me strange, thus into different sizes I will change
When the day after tomorrow is yesterday. Today will be as far from Wednesday. As today was from Wednesday. When the day before yesterday was tomorrow. What is the day after this day?
What goes through a door but never goes in. And never comes out?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?
People take me on walks and I run and have fun. What am I?