Freedom, generally, is having the ability to act or change without constraint. Something is "free" if it can change easily and is not constrained in its present state. In philosophy and religion, it is associated with having free will and being without undue or unjust constraints, or enslavement, and is an idea closely tied with the concept of liberty. A person has the freedom to do things that will not, in theory or in practice, be prevented by other forces. Outside of the human realm, freedom generally does not have this political or psychological dimension. A rusty lock might be oiled so that the key has the freedom to turn, undergrowth may be hacked away to give a newly planted sapling freedom to grow, or a mathematician may study an equation having many degrees of freedom. In physics or engineering, the mathematical concept may also be applied to a body or system constrained by a set of equations, whose degrees of freedom describe the number of independent motions that are allowed to it.