Penny for your thoughts. When this saying was made up, you could actually buy something for a penny, Does that make cents?
Think of your brain as an empty void universe as you age there are more and more thoughts moving randomly though your brain. The more thoughts means there are more collisions of your thoughts When this happens and the thoughts are of equal size, then and idea occurs. If one thought is extremely massive, it wipes out the other thought completely. Thus accounting for senile moments.