There was recently a storm near me. T was a lightning storm and I was wondering if it really is true about it only hitting tall things or if it is really just an old wives tale. If it is true why is it that way.
Lightening strikes from a point of excess electrons (negatively charged) to a path to a deficit of electrons (positively charged).
As a rule, lightening usually strikes from the clouds to the ground, but occasionally, the opposite can happen as well.
It is not always the case that the tallest object will be struck. The path which offers least resistance to ground will be struck, and that may not necessarily be through the tallest object.
The reason is that the tallest object might be in the path along which lightning finds to be a less attractive path to ground, or an alternative path than the tallest object might be closer.
As a hypothetical example, if there was a rubber tree that stood 100' tall, next to a live tree which stood 50' tall, the taller tree has less of a chance of being struck, because it is negatively charged (has excess electrons), while the shorter tree is positively charged (has a deficit of electrons) thereby creating a better path to ground. Thus, the live tree - while shorter - is the more dangerous place to be.
For this reason, tall objects are "grounded". This means, there is material at the top of a tall object - like a building - connected to the ground by means of a wire or structure. What will happen is lightning will find the route to ground to be more attractive by going through the wire or structure, rather than through the building. That is why the building you may be in can be struck, and you won't be hurt.
There are a few parallel examples in life which illustrate this phenomenon.
One is the example of a person parking their car in a parking lot. Two cars parked next to each other present an opportunity for either to be broken into - or stolen. The car which offers the best opportunity to be broken into for its contents will be the mark - and that could be both cars. The car which offers best opportunity to be stolen will be the mark - and that will usually be one car. If you have a convertible, best not park next to another convertible, since both cars are more attractive to a thief looking for cars or contents. Best is to park close to the store, or near a security camera. If you drive a Mercedes, best not to park next to a Ford Pinto unless you park close to the store. The safe bet is to park close to the store. A thief will find an easier mark, just like lightning will choose a path of least resistance.
The other example is oddly apropos to this group: How does one outrun a bear? The answer is: you don't have to outrun a bear, you need only run faster than the person next to you. The slower person will be more attractive to the bear than you.
Lightning likes to take the path of least (electrical) resistance. Air has an extremely high electrical resistance, so lightning will strike just tall objects so it doesn't have to travel as far through air to get to the ground (even things we think about as insulators, like wood, conduct electricity much better than air)
To get to the point of your question: Lightning doesn't only strike tall things, but it does strike them preferentially. If there's nothing tall around (like in the middle of a lake for example), it will still strike
@Wigwam's answer is a pretty good explanation, but I thought I would add to it a bit more "common" one.
First lightning is extremely powerful, and it does not need to hit you to totally mess you up or kill you.
Lightning will hit just about anywhere, though it likes to take the "best path" to the ground. That's not technically true, but like any electricity, it's trying to get to the place with the most difference in electrons. However, on a planetary scale, that's usually the ground (if your standing on the ground, it's different if you're in the air).
Tall things offer a better path to the ground than the air.
Metal offers a better path to the ground than other things.
Humans are a crappy conductor but were not the worst.
So if you're stuck outside in a thunderstorm, you don't want to be the tallest thing for miles, nor do you want to be near the tallest thing for miles. Other then that your pretty safe, EXCEPT, that many buildings and man-made structures have lightning rods (metal rods that are well grounded by a large wire or strap) and will actually attract lightning. Remember you don't have to be hit by lightning for it to suck. So you don't want to be around those either. Best advice is to stay out of the rain and stay away from obvious targets. If there is a lot of lightning in your area go inside. If you can't go inside, pick small trees and bushes over tall trees to stay dry under, and don't do silly things like huddle under a cell tower.