The chicken or the egg causality dilemma is commonly stated as "which came first: the chicken or the egg?".
The dilemma stems from the observation that all chickens hatch from eggs and all chicken eggs are laid by chickens. "Chicken-and-egg" is a metaphoric adjective describing situations where it is not clear which of two events should be considered the cause and which should be considered the effect.
Here you have a nice scientific explanation:)
Source: Sketching Science fb