Do we all actually just weigh half of what our bathroom scales tell us? Here's the reasoning: Newton's third law tells us that every force has an equal but opposite reaction force. So besides my weight pushing down on the scale from above, there's also an equal force pushing up on the scale from below, so the spring or pressure-sensor-thingy inside the scale is being squeezed from two directions by double the force! That's tipping the scales, and therefore everybody is actually just half as heavy as it says on the readout.
 A: You've inserted the scale between yourself and the planet Earth.  The scale's spring is compressed by the force exerted on you by Earth's gravitation.
The compressed spring tries to uncompress itself and pushes back on you with linear restoring force.  Linear restoring force is exactly equal to the force exerted on you by the Earth's gravitation.
By recording its linear restoring force on a scale, the spring measures the force exerted on you by Earth's gravitation.  The distance the spring is compressed will depend on its stiffness, which is measured by Hooke's Law, which says that distance compressed is directly proportional to the stiffness of the spring.
Force applied to the spring = + spring stiffness constant * distance of compression
Force applied back to you = - spring stiffness constant * distance of compression
The spring stiffness constant is a dimensionless number calculated according to Hooke's Law.
The net result is that the spring measures BOTH the force you apply to it, and the force it applies to you.  But as the measure is exactly the same and measures the same quantity, there is no need to halve it.
Here is a graphic explanation, which also discusses the potential energy stored in a compressed spring: http://theory.uwinnipeg.ca/physics/work/node5.html 
