Subroutine is an instruction sequence in a machine or assembly language program that can be prewritten and referred to as often as needed. Subroutine is used for controlling thing e.g. traffic lights burglar alarms they all use subroutine
A stack can be used to implement a subroutine call because the return address is pushed onto the stack, and control transfers to the subroutine. If the subroutine were to then call another...
A model aeroplane is used for presentations and testing. Model planes are also used for fun. They are used model-plane clubs, aeroplane company headquarters and studios. (Bedroom, in my house)
The only answer that comes to mind is "Airships". Hydrogen, the lightest element, was used in airships, where excess weight was undesirable. However, accidents like the Hindenburg disaster showed...