First Law of Thermodynamics

The first law of thermodynamics states that total energy of the universe is conserved i.e. energy can neither be created nor destroyed. 

For a closed system    ΔU=Q+W

for surrounding           ΔU=-Q-W

For universe               ΔU=ΔU(system) +  ΔU(surrounding) = 0

The total energy of universe is constant. It seems to be simple, but it strikes out so many impossibilities. Like the one I had in my childhood, I had a dream to build a device that will produce continuous power by rotating a wheel, which will drive a dynamo to produce electricity, part of which will be utilized to drive wheel and rest can be used for other purposes. but after studying this law I got to know, it is not possible to make such a system.  

We can't produce energy without giving it some equivalent amount of energy in another form.