Why does my refrigerator door resist opening? After closing my refrigerator's door I noticed that it's much harder to reopen it immediately, as if there's an underpressure. Only after a minute or so it opens normally. How can this be explained?
 A: What @stevenh said.
I always look for that resistance to make sure that the door has really closed when I overfill the refrigerator. If yours equalizes after a while air must be leaking in, look at the seals around the door if they have deteriorated. Mine is fairly resistant once closed well, both the freezer compartment and the normal one. 
A: When you open the door the cold air flows out and is replaced by air at room temperature. When you close the door this air gets cooled through contact with the stuff inside the fridge and therefore the pressure decreases, according to ideal gas laws by some 10%. That's not very much, but the surface of the door translates this small pressure difference to a larger force.
Note that the effect is even stronger with deep freezers, where the difference in temperature (and hence pressure) is even bigger.  
edit
Like Anna says there must be air leaking in if the door opens normally after some time. This air will be cooled as well, but the pressure will increase with more air being sucked in, until an equilibrium is reached: cold air at atmospheric pressure.
A: many modern refrigerators have a mechanism that pumps out air from the fridge after you close the door, to save money on cooling the region.  This creates an immediate pressure drop.  Of course it can't be maintained forever, but it still improves the energy efficiency.
A: There are magnets in most fridge seals.
A: the door has magnetic lock inside the seal to close.check out by placing a piece of iron on the seal.but remove it while closing.when the door is closed, the seal around it compresses and closes all gaps from air entering or loosing.when we try to open it, actually the seal expands to original shape and creates no gap until an inch or so thereby a under pressure is generated which required more force to open.
