Voltage reading between hand and 3.3V source is only 0.1mV. Why? I held one multimeter probe to a 3.3V source, the other probe I held in my hand. The voltage measured between these 2 points was around 0.1mV. How is this possible? I expected 3.3V since I thought my hand was a ground?
What I find at least as strange is that when I held one probe in my left hand and one in my right the voltage is larger than the former measure(around 2.3mV!! But it fluctuates much more though)
Edit (After reading Olin Lathrop's answer and reading about voltage drop)
Is the following correct?
So the multimeter can't pick the real voltage drop up because the total resistance is too high but for some reason related to how the multimeter is built picks up a stable 0.1mV.
Also, the circuit extends from my hand to the floor I'm standing on to the table's legs up to the ground terminal of the device delivering the voltage. The real voltage drop between the two probed points is very small because the resistance of the multimeter dissipates so little energy it hardly drops the voltage.
Suppose the multimeter was a perfect measuring device, given that the larger the resistor, the more energy used by that resistor, and the bigger the voltage drop across that resistor and my body and the rest of the circuit has a very high resistance, the voltage drop between my hand and the ground terminal is actually very large.
 A: Draw a complete schematic of what all was connected and you should be able to see for yourself.  The negative terminal of the power supply and your body were not connected.  For example, they weren't both connected to ground via a sufficiently low resistance path.  Just because a power supply uses wall power and has a ground lead on the plug doesn't mean any part of the output is connected to that ground.  Most ordinary power supplies like that, and all "wall wart" type supplies you will be able to find are deliberately isolated from the input power feed for safety reasons.
There are two reasons you measure some voltage when holding both meter probes in opposite hands.  First, the moisture of your skin and the probe metal will cause a small battery effect.  It is very difficult to make the two battery effects of each hand ballance out so that the meter reads zero.  2.3 mV is really a very small voltage.
Second, your body is most likely picking up a significant amount of power line hum, which becomes a common mode signal to the meter.  The meter's electronics aren't perfect, so some of this common mode signal can show up as a differential signal, or get rectified a bit to show as a DC offset.
Lick the finger holding the meter probe of one hand only, and you should see a larger voltage between the two hands.
