Pandolfini's Puzzler #9 - Six Hard Moves

“Occasionally,” Professor Pando began, “I lighten up and pose easy problems anyone can solve, even a novice. Cast your eyes on the following position.” 
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White to move
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Question: How can White checkmate Black in six moves?” 
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“Six moves?” Zephyr asked. “That’s not an easy thing to do, to look six moves ahead. There are so many possibilities.” 
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But then Zephyr really began to look at the position. And she noticed something -- and she chuckled. 
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“Oh, the problem is really very easy.”
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What did Zephyr mean? 
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Answer below - Try to solve ProfessorPando's Puzzler first!
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ANSWER #9
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Zephyr soon understood that all six moves were forced (except Black’s 5th move). 
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The variation Zephyr found was: 1. d4 b5 2. d5 b4 3. axb4 a3 4. b5 a2 5. b6 a1=Q (or Black could underpromote as well) 6. b7 mate!. 
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Take note
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The present example shows that the difficulty of problem is not solely based on its length. A short problem can be very hard and a long one can be very easy.
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