How do turnadoes form?

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I am curious as to how twisters are formed. Why do I seldom see it here in the Philippines. Most of the turnadoes I hear about are in the US and mostly on flat land. What's the rationale of going to a lower level of the ground or a depressed part of the land to protect oneself from the twister. Why can't the turnado suck up anyone hiding from the narrow depression part of the land?