My nephew (a high school student) asked me last week how time constants work. Time =resistance X capacitance. T=RC

He was please with this simple explaination:

T=RC T= VOLTS/AMPS x COULOMBS/VOLT

T= JOULES PER COULOMBE/COULOMBS PER SECOND x COULOMBS/JOULES PER COULOMBS

Reducing this shows that T=Seconds