Every year around 1 billion Valentine cards are sent. After Christmas it's a single largest seasonal card-sending occasion.
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Kissing releases masses of oxytocin, a "love potion" that helps couples bond and feel more attracted to each other.
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Parents receive 1 out of every 5 Valentines.
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In medieval Italy, if a man was caught kissing a woman in public, he had to marry her whether he liked it or not.
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Falling in love raises levels of nerve growth in a person's brain for about a year and can improve their memory.
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In Korea, there's a man named Lee Jin-gyu who fell in love and married his life-sized Japanese body pillow (called a "dakimakura").
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Couples' personalities converge over time to make partners more and more similar.
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