Can touching a single power line (on the street) kill you? Statement: You need to touch two power lines at the same time to get electrocuted. I am kinda doubting this now because, in the video linked below, it looks like the person is only touching one line but still gets an electric shock. Is this possible? and also the person is on a tree and aren't trees insulators?  This video isn't sensitive or anything, it actually funny, so please don't hesitate to watch. Thanks!
Video
 A: Unless there is sufficient insulation (electrical impedance) between you and the earth  when you touch a high voltage wire, yes you may get electrocuted. This is because most electrical power systems in the world are earth grounded (referenced to earth). The higher the voltage the greater the impedance to between you and earth ground needs to be. At 60 Hz it only takes 50-100 mA of current in a path through the heart to cause ventricular fibrillation.
If you are isolated from ground and touching two wires with your bare hands, electrocution is possible if the voltage difference has the potential to cause a lethal electric shock and the insulation on the conductors is insufficient to limit the current to below the threshold for a lethal electric shock.

oh okay, bats die when they hang on the cables, but they do not
  provide any connection between live and ground, how is this possible?

Regarding your above comment to Solar Mike, it is ONLY possible for the bat to be electrocuted if the bat is simultaneously in contact with the high voltage wire and another wire or grounded object where the voltage between them has the potential to cause a lethal electric shock.
Bats are physically different than the typical birds you observe on HV wires. For one I understand they may have wingspans much larger than ordinary birds. So if they are on a HV wire and spread their wings, there is a higher likelihood that their wings will simultaneously contact more than one HV wire at a time, or some grounded part, causing lethal electric shock. Regardless of the reason for their deaths, they must simultaneously be in contact with two conductors with a potential difference capable of causing a lethal electric shock.
Hope this helps 
A: If you touch the wire and the ground at the same time, you close an electric circuit. This electric circuit starts at the power station, goes through the wire, through your body and into the ground. There are multiple parallel paths from there back to the power station, either directly through the ground to the power station or through the ground to one (or multiple) near power poles with grounded wires, which closes the circuit, such that you get shocked. 
If you only touch one power line without connection to the ground, you don't get shocked, like birds sitting there.
If you touch two wires at the same time, you will get shocked if they have a difference in potential. On a common power line there are three wires for three different phases. Since each wire has a phase shift relative to each other wire, you will also get shocked if you touch any pair of wires.
There may also be one or more ground wires, which means you will not get shocked by touching one of these and the ground (since they are connected anyways), but touching the ground wire and any of the three phase wires will shock you.
A: Watch this video to realize that power lines,  are dangerous for  big birds, who span two of them at a time , and get electrocuted. Also fires may start this way, the bird may catch fire and transfer it to the dry grass underground.
You have to realize what 3 phase power lines are. One wire carries no current and the other three each have a different alternating current phase. Thus, if any of the current carrying wires is shorted (the bird's body is a good conductor) with the neutral, a current passes through the bird and kills it.
For the boy in the video, his stick touched a current carrying wire and he made a short with the earth, but fortunately the stick was a bad conductor, so not much current passed.
If it were the neutral wire, again it would not be so dangerous, although some voltage might be built up in the distance from the factory to the point of contact. That is why the home outlets have three pins for safety: the live phase, the neutral, and the ground of the house .
A: Yes, Trees are good insulators, and sometimes grounding is ignored if power lines are installed on trees. But still, they can break, and this phenomenon is called Electrical breakdown. this happens on high voltage, and high voltage is just what power lines are sending. So yes, current can flow, and if it flowed through the heart, you can even die.
