Differentiate between compound and mixture.

Compound can be describe by a chemical formulation, but
mixture can't
for instance, we can say H2O is a compound, but we can not say sugar water is a compound but mixture

A compound is combined chemically, while a mixture is not chemically combined. 

The  grandmother cell  is a hypothetical  neuronthat represents a complex but specific concept or object.[1]  it activates when a person "sees, hears, or otherwise sensibly discriminates"[2]  a specific entity, such as his or her grandmother. the term was in use at least as early as 1966 amongst staff and students in the department of experimental psychology, university of cambridge, england. a similar concept, that of the  gnostic neuron, was proposed two years later