Answer:
What does a WW1 Trench, A Submarine and a Popular Phone App all have in common?
Round like a dishpan and smaller than a bathtub. But the ocean can't fill it. What is it?
Look into my face and I'm everybody. Scratch my back and I'm nobody.
What surrounds the world, yet dwells within a thimble?
What gets wet while drying?
Sometimes I am white, sometimes I am dark. Most people love me. What am I?