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A rubber suker creates a seal between the surface and the suker, it hold because in order to remove it one has to temporarily create a vacuum and this requires a lot of energy. On a rough surface an airtight seal cannot be made as air can escape under the suker, and so no such vacuum can be formed and the suker does not stick.
The suker is dish shaped when pressed against a smooth surface the air is forced from beneath the suker. the rubber makes an air tight seal and the air pressure outside is greater than the air pressure beneath the suker, thus forcing the rubber suker to stick. Same reason a syringe is filled with substance. 

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