For a simple interface like a web-browser displaying junk of text, we need a lot of things unchanging - giving the constancy feeling which is very essential for us to "do" anything. How much does the universe have to "do" to give us the constancy feeling that we so much enjoy. "The only thing that does not change is change itself" - is a well known observation by some random old dude many centuries ago, though the constancy feeling that does enable us to observe and appreciate the change is equally (if not more) important. So what does the universe maintain constant for us to enjoy life? Which cardinal constraints should all physical and chemical reactions follow?
1) Energy: Energy is not lost or gained but remains constant in any reaction. Though, if you very small particles (quantum stuff), energy and mass are constant (because Einstein says energy and mass are interchangable).
2) Momentum and Angular momentum: Take a particles mass multiply it by its velocity (or angular velocity) and you get the momentum. Do anything (walk, jump, cry or die), you should conserve momentum as badly as you should conserve energy. It so happens, even the small stuff inside the atom needs to conserve momentum and angular momentum.
3) Charge: Charge is a property shared by all particles. +ve, -ve or neutral. Once a particle has a charge, it cannot magically loose it. In any type of reaction, if a charge comes in, one of the outputs should take the charge out. So the overall charge is same.
4) TCP: time reversal, charge conjugation and mirror reversal. This conservation happens only in quantum stuff. Take any quantum reaction,
a) take the outputs of the reaction and run the reaction backwards (time reversal)
b) Conjugate the charges (electron to positron etc..).
c) Run the reaction in mirror image. Every particle spins clockwise or anticlockwise, just swap them.
If you do all these, you should get the original inputs back! - theoretically of course.
These are the only things that are always there and give us the continuity feeling that we are so used to. So wat? Nothing, just typing junk as usual. Go ahead and do some real work instead of wasting time on pointless stuff on the net.