Every second, a green marble has a chance to appear. You are not told what the probability of this occurring is - instead, you are given a number which is equal to the average number of seconds it takes for a green marble to appear (let's call it 'mean time to happen').
Now let's say you want there to be a chance of a red marble appearing instead of a green one. In order to make this happen, you must define a separate 'mean time to happen' for the red marbles. But you do not want to change the overall chance of any marble to appear; the combined probability of a red or green marble appearing must be the same as the previous probability of a green one appearing.
Aim at, say, a fifteen percent probability of a red marble appearing instead of a green one, with the current 'mean time to happen' being one thousand seconds. What 'mean times to happen' would you have to assign to both green and red?