How come Wifi signals can go through walls, and bodies, but kitchen-microwaves only penetrate a few centimeters through absorbing surfaces? The only difference that I know of between kitchen microwaves and WiFi signals is how much power is pumped through them.
Why is it that WiFi signals, being  1000 times weaker can travel so much further, while something like a concrete block can absorb $1000 \, \mathrm{W}$ of microwaves from a kitchen magnetron being blasted at it? Or am I mistaken about the materials that WiFi signals can pass through? 
 A: The big difference is that you want to cook with microwave ovens and to communicate with WiFi.  Microwave ovens are allowed to leak up to 5 mW/cm^2 measured two inches away per the FDA standard.  For WiFi he EU allows 100 mW EIRP, which is less than the oven could leak if it leaked evenly in all directions.  Communication devices are built to work over a wide range of signal levels, so even if there is attenuation going through the walls you still get WiFi.  The oven leakage is attenuated just the same, but we don't pay attention to it.  The power level is far too low to cook with.
A: To start with, the microwave oven is a Faraday cage. Only objects inside get the full power of the magnetron. As far as I know, the microwaves penetrate all the way through objects in the oven, although the microwaves are not evenly distributed (which is why there is the rotating plate).
The WiFi is meant to broadcast the signal in all directions. Although, larger microwave antennas are directional and use line of sight.
A: Just like light, some materials absorb microwaves, some materials reflect microwaves, and some materials transmit microwaves.  You may have noticed this: when you put food in the microwave, some parts of it get very hot, and other parts of the food are still cold.  Some of this is due to the uneven distribution of microwaves in your microwave oven, but this is also due to the fact that certain materials just don't absorb microwaves very well.  For example, put a mug of water in the microwave--the water might be boiling hot, but the mug will be cool enough to hold in your hands.
One of the reasons why microwaves work so well at heating food is because food contains water, which absorbs microwaves.  If you made your house out of water, your WiFi wouldn't work very well.  The same thing happens if you use a lot of reinforced concrete--the rebar reflects microwaves, and you get bad WiFi reception.  But most homes have walls made mostly out of air, with a little bit of gypsum, paper, and paint in front of the air.
If you ran a microwave oven with the door open, it would send a "signal" that would travel pretty far.  You would probably piss off the neighbors.  Maximum WiFi power is around 1 W, so 1500 W of power would travel ~40 times as far.
This is actually the reverse of how the microwave oven was invented--an engineer named Percy Spencer was working with a radar set, and noticed that the radar set was melting his candy bar.
A: Less than a year after this question was originally asked, astronomers discovered a source of mysterious signals that many believed to be from space: microwave ovens.  Turns out someone in the observatory was opening up the microwave too soon!  Seriously:  The Mystery Of Fast Radio Bursts Is Solved, Forbes (2016-02-25).
As the other answers have said, it's that microwave ovens are designed to keep the microwaves in pretty well.  Just thought that this funny story was a good illustration of that concept.
A: *

*the absorption, reflection and transmission of radiation through matter is actually wavelength dependent.

*the frequencies used for oven and the wifi or mobile network are different.


*

*the confinment of microwaves inside the oven is not die to absorption of the oven walls. Rather, it is die to the high reflectivity of the wala that does not allow microwaves to transitie through.



I haven't experimented this, but i beleven that if you put your mobile with in microwave oven, it shouldn't catch signals.
