How do metallic objects interfere with X-Rays? During my visit to the doctors', I recollect having seen the sign which says 

Please remove all metallic objects from the body before the commencement of the X-Ray

How do metallic objects interfere with X-Rays? Don't they have metallic implants for certain kind of surgeries?
 A: It is well-known that x-rays are blocked by metal. [Ref: Kid's Science]
Obviously the doctor wants to look at your internal organs, unobscured by a fuzzy outline of your house keys and pendant.
So, the sign is requesting that you removing metal items from the external parts of your body, to allow visibility to the internal parts.
(MRI is a totally different story.)
A: To add a slightly more detailed answer than yesterdays kids's answer:
X-rays are electromagnetic waves, like light. They're different from visible light because they have a higher frequency, and therefore a higher energy per photon. That also means they have a lower wavelength (since f*λ=c)
Metals are solid materials in which some electrons are not strongly bound to a single nucleus, but instead form a electron cloud dispersed throughout the whole object. These electrons can therefore move easily, which is why metals conduct electricity.
An x-ray photon that hits a metal object may have enough energy to shoot straight through. If not, it will hit an electron and kick it away at high speed. The energy and momentum is taken from the x-ray photon, which means it's scattered in another direction at lower energy (and could then be absorbed outright).
So, it's the metals electron (band) structure that causes the interference with x-rays.
