Every answer from Classic Riddles Set 3, with an explanation for each. The two hard ones both turn on noticing what a word is doing rather than what it means.
What has teeth but cannot bite?
Answer: a comb
A comb's teeth are the row of thin prongs that run through your hair.
What has four legs in the morning, two at midday and three in the evening?
Answer: a person
A baby crawls on all fours, an adult walks on two legs, and in old age a walking stick makes three.
What can you catch but never throw?
Answer: a cold
You catch a cold from someone else, but there is nothing solid to throw back.
What goes up but never comes back down?
Answer: your age
Age only ever increases, no matter how much anyone might wish otherwise.
What can fill a whole room without taking up any space?
Answer: light
Light reaches every corner of a room, yet it has no volume and displaces nothing.
I am always in front of you, but I can never be seen. What am I?
Answer: the future
The future is permanently ahead of you and never visible, because the moment it arrives it becomes the present.