Good Riddles

Question: When is the hardest time to catch a frog?

Question: You have bag, you put a lump of metal into the bag, then you put a lama into the bag, then you put something else into the bag, and yet the bag weighs nothing. Why?

Question: I can be one, I can be two. I can be Open, Closed, Empty or Full. I can be Bare, but never a Bear. What AM I??

Question: in all of written history I won't show up at the end of a sentence but I will be there in the end . . .

Question: Act in my name, and thus remove me. What am I?