Karma

"The word "karma" from the sanscrit root 'kri' signifies action or deed, whether physical or mental (including thought).[...] Karma means not only action, but also the result of the action. the consequence is really not a separate thing, but is a part of the action. [...]
There is no such thing as a blind chance or accident. If your finite mind is not able to find the cause of an event, it does not mean that this is accidental. [...]

If there is an action, there must be a reaction. The reaction will be of equal force and of similar nature. Every thought. desire, imagination, sentiment cause reaction. Virtue brings its own reward; vice brings its own punishment. This is the law of reaction. God neither punishes the wicked nor rewards the virtuous. It is their own karma that brings reward and/or punishment. No one is to be blamed"

 Swami Vishnudevananda "Meditation and mantras"

In many languages there are the quotes about action and reaction ex. ''behave with others as you want them to behave with you"(original is from Bible , Luke 6.31). It is very commom thought, and everybody knows that if we are nice to our neighbours so they will be nice to us, if we will help them, they will be helpfull to us. But how many people know that this law concerns also our thoughts? It is not enough to smile to our neighbour if in our heart we wish to him something bad. Thought is powerfull as the action is.

Karma law is easy: good action, good word, good thought- good reward, bad thought, action or word- punishment. You can think that nobody knows our thoughts, of course word or action are hurting very easy other person, but our bad thought can hurt ourself later.

It is easier to get control on the body and words then on mental activity, but once we get control of the mind we dont need to worry about the body. Everything is starting from the thought.

Om Tat Sat