How come?
phrase

- used to ask about the reason for something (why?)

(Cambridge Advanced Learner's Dictionary)

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DID YOU KNOW?

The Random House Dictionary says "how come?", a colloquial Americanism for "how is it that?" or "why?", is based on a shortened form of an expression such as "how comes it that..." or "how did it come about (that)..."

These still-somewhat-obscure phrases depend on a usually archaic sense of come, meaning to come about, happen, occur or become in combination with an archaic word order that places "how" and "come" together at the beginning of a sentence:

HOW COMES it then that he is prince of devils? (Marlowe, Doctor Faustus)

HOW COMES this about; there must be some mistake? (Jane Austen, Mansfield Park)

The modern form eliminates the "s" and is used informally to pose a question. It can even be used in important questions such as:

HOW COME anything you buy will go on sale next week? (Irma Brombeck)

If truth is beauty, HOW COME no one has their hair done in a library? (Lilly Tomlin)

If it's natural to kill, HOW COME men have to go into training to learn how? (Joan Baez)

HOW COME there's only one Monopolies Commission? (Nigel Rees)

HOW COME you never see a headline like "Psychic Wins Lottery"? (Jay Leno)

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SYNONYMS

Why, why is it…, why is it that…, how is it that…

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PRACTICE OWAD TODAY
Say something like:

"HOW COME that toast invariably falls butter-side down?"