The riddle
What Can Travel Around the World While Staying in a Corner?
The envelope in the picture
Envelopes carry a postage stamp in one corner, and the envelope goes wherever the address says. The stamp never leaves its corner, yet it crosses countries and oceans along with the letter. Said that way, both halves of the riddle are true at once: the traveling belongs to the letter, and the corner belongs to the stamp.
The first one
The world's first adhesive postage stamp used in a public postal system was the Penny Black, issued in Britain on May 1, 1840 and valid for postage from May 6. It showed a profile of Queen Victoria and cost one penny, which paid for a letter weighing up to half an ounce to travel anywhere in the country. Before it, the person receiving a letter usually paid for it on delivery. Rowland Hill argued for paying in advance, with a small label on the letter as proof, in a pamphlet printed in 1837.
What a stamp proves
A stamp is a receipt that travels with the thing it paid for. It tells the sorting office that the postage is settled, so the letter can go. Collecting them has a name of its own, philately.
Other places to stay put
Plenty of riddles pair a journey with an object that does not move. Compare going up and down but never moving, which follows the same pattern.