The second law of thermodynamics
I am not a physicist, but I will try to explain what I understand of it. If there is any scientist over there that wants to give me some help in explaining this he or she is welcomed.
This law states that thermal energy (heat), is special among the types of energies. All the forms of energy can be converted into heat, but in a way that is not reversible. It is not possible to convert the heat back fully in its original form. This essentially means that in each reaction, there is some energy that is going to be lost as heat without converting into another "more useful" form of energy. This unusable heat is characterized by the random motion of molecules. The second law says that the amount of random movement(entropy) can only increase in a closed system. Being the Universe a closed system, the law says that entropy will always increase in the Universe. This means that the random motion will only increase with time. After billions of years the possible scenario is the even distribution of heat (random motion) throughout the Universe.
Many people have tried to imply that the evolution contradicts the second law of thermodynamics. The Earth is an open system, receiving continuous new energy from the sun. The only way that life has been getting more complex in our planet as time passed is because the energy dissipated by the chemical reactions of life is replaced in excess by the sun' energy.
Links: The second Law of thermodynamics, Entropy