A source of definition difficulty in relativity arises from two definitions of mass in common use, one of which is formally equivalent to total energy (and is thus observer dependent), and the other of which is referred to as rest mass or invariant mass and is independent of the observer. Only "rest mass" is loosely equated with matter (since it can be weighed). Invariant mass is usually applied in physics to unbound systems of particles. However, energies which contribute to the "invariant mass" may be weighed also in special circumstances, such as when a system that has invariant mass is confined and has no net momentum (as in the box example above). Thus, a photon with no mass may (confusingly) still add mass to a system in which it is trapped. The same is true of the kinetic energy of particles, which by definition is not part of their rest mass, but which does add rest mass to systems in which these particles reside (an example is the mass added by the motion of gas molecules of a bottle of gas, or by the thermal energy of any hot object).
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What type of systems is rest mass applied to?
Ah, so.. unanswerable

Football is the most popular sport in Portugal. There are several football competitions ranging from local amateur to world-class professional level. The legendary Eusebio is still a major symbol of Portuguese football history. FIFA World Player of the Year winners Luis Figo and Cristiano Ronaldo who won the FIFA Ballon d'Or for 2013 and 2014, are among the numerous examples of other world-class football players born in Portugal and noted worldwide. Portuguese football managers are also noteworthy, with Jose Mourinho, Andre Villas-Boas, Fernando Santos, Carlos Queiroz and Manuel Jose among the most renowned.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What is the most popular sport in Portugal?
Ah, so.. Football

The Canadian Army is headed by the Commander of the Canadian Army and administered through four divisionsthe 2nd Canadian Division, the 3rd Canadian Division, the 4th Canadian Division and the 5th Canadian Divisionthe Canadian Army Doctrine and Training System and the Canadian Army Headquarters.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): How many divisions are there in the Canadian Army?
Ah, so..
four divisions