Exhausted...

I am not a young person anymore. I only watch Cash for approximately four hours a day and part of that time he is napping but I am still exhausted when I get home. How can anyone so little tire someone out so quickly?? It sounds like I am complaining but seriously I am not. He is a very good baby and only fusses when he is hungry, tired or his gums hurt him. The dog is more of a nuisance than little Cash...