Excelsior (chess problem)

"Excelsior" is one of Sam Loyd's most famous chess problems, originally published in "London Era" in 1861.
In 1867, it participated together with five other problems as a set in an international problem tournament. The motto for the full set was "Excelsior" (eng. 'Ever upward'), generally known as the title of the poem "Excelsior" by Henry Wadsworth Longfellow, and as that term is very fitting for this particular problem, it is generally associated with it. 
Backstory.
Loyd had a friend who was willing to wager that he could always find the piece which delivered the principal mate of a chess problem. Loyd composed this problem as a joke and bet his friend dinner that he could not pick a piece that "didn't" give mate in the main line (his friend immediately identified the pawn on b2 as being the least likely to deliver mate), and when the problem was published it was with the stipulation that White mates with "the least likely piece or pawn".
It should probably be noted that the first publication, in 1861, is not accompanied by any such stipulation.
Solution.
1. b4!
1... Rc5+ 2. bxc5!
2... a2 3. c6! 
3... Bc7 
4. cxb7 "any" 5. bxa8=Q/B#
Any problem that features a pawn moving from its starting square to promotion in the course of the solution is now said to demonstrate the "Excelsior" theme. Nowadays it is most usually shown in helpmates and seriesmovers.