Dear kind woman at the gym:
Thank you for giving me the nice compliment as you were leaving today. It wasn't much, but it made my morning! Being in the locker room at the gym is always a little intimidating. I'm not a size 0 and my body jiggles in places that it probably shouldn't. Previous to your kind comment I had been thinking how much I would kill to look like you. Not overly skinny, but toned, muscular and fit. It's nice to know that even people that look like you sometimes admire things about me too!
On my way out I complimented the woman who was watching my daughter. I hope that she passes on a compliment to someone else during the day. See what you hopefully started? A chain of sunshine, if that doesn't sound too cliched. So thank you, again, for being so nice. I can only hope someone tells you how great your hair looks too.