capacitor - RC circuit as clock source

I am not understanding something very fundamental here , I have read that an RC circuit can be used as a clock source. From this I assume a DC input. Playing around with my oscilloscope the only time I see an RC circuit oscillate is with an already oscillating input, throwing in a DC input there is no oscillating just the obvious blocking of DC by the cap, so how does an RC circuit work as on oscillator with a DC input? And I know you can make fancy ones with more components but I mean a very simple resistive/capacitive circuit? Thanks in advance.