How do I the screen on a Nokia Asha 300

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So I got a new phone with my contract, it's got a touchscreen as well as the number pad. To unlock it you simply press the power button the stroke the screen from left to right, but when I put it in my pocket I keep finding it on random menus or logged into the web (great!). Does anybody know how to lock the screen, Ican;t find the answer onthe nokia web pages or in the manual.