They act as giant petri dishes, where creative types and entrepreneurs rub up against each other, combining and recombining to spark new ideas, new inventions, new businesses and new industries.

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Both for what it says about changing attitudes towards aid in Africa, as well as the more familiar chaffing that occurs when pride, philanthropy, poverty and celebrity rub up against each other.

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But its introduction has contributed to outbreaks of violent conflict between Muslims and Christians, especially evangelicals, in the middle belt of the country where the two communities rub up against each other, in cities like Kaduna and Jos.

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As China continues to rub up uneasily against its maritime neighbors, including Japan, which occupies another set of disputed islands, these seas could get stormier.

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