Answer:
There are several different kinds, but the one you pick doesn't do its job. What is it?
What does a WW1 Trench, A Submarine and a Popular Phone App all have in common?
As round as an apple. As deep as a cup. All the king's horses can't pull it up.
A harvest sown and reaped on the same day in an unplowed field. Which increases without growing, remains whole though it is eaten within and without. Is useless and yet the staple of nations.
What is neither inside the house nor outside the house but a necessity for any home?
What language does a billboard speak?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?