wat is the basic delay circuit how do we design them

when i attend my first interview the person who was taking interview to me asked the same question.can you design a basic delay circuit.?

Here is the answer.We all know that a basic delay circuit can be constructed with R and C combination.

But it will act as a delay circuit.

I think this gives some idea about it.

when we say
RC = T(time constant)
it is because R*C = (V/I)*(Q/V)
now R*C = Q/I
we all know that I=Q/t
so R*C = t (time constant).

hence we can design as simple delay circuit.