Opening tight bottles Why do so many people recommend patting/hitting on the lid of bottles, or alternatively on the bottom, in order to open them when they are too tight? Is it even effective? And why would it work?
 A: If you hit the bottom of a jar while it is upside down, it loosens the contents and piles them on the lid.  If the inside pressure is less than the outside pressure, which should always be the case if the jar was vacuum packed, this may provide a little extra pressure to counteract the outside pressure that is holding the lid on.
Tapping on the bottom may also release water vapor and/or other gas from the contents (like shaking a soda bottle), which will help counteract outside pressure. The same effect may result from hitting the lid, with the added bonus of deforming the lid slightly and breaking the vacuum seal.
But the safest way to loosen a tight cap may be to run hot water on it so that it expands slightly.  Glass has a lower thermal expansion coefficient than metal, so this method usually works.
A: Whacking on the lid of a bottle, e.g. with the handle of a knife, works because it disrupts the seal that is holding the cap on. 
A: Most bottles (plastic ones, at least) I've seen attach to their caps with a screw like mechanism. I think a screw like mechanism can get jammed easily if one of its components is out of place. Whacking or knocking on the cap could help dislodge the bottle/its cap from an out of place position to one where it is naturally supposed to be, which would make it easier to open.
This, of course, is true for a jammed bottle. I'm not sure if the same would work for "tight" ones.
