What's a Mom to do???

So today I found 2 of my kids (I'm sure if #3 could figure out how she would have joined in) jumping on my bed, for the 199th time. And for the 199th time, I told them not to. And then I proceded to spank their little bums because of the 199 warnings they had previously had. I spanked #2 first, and she took it quite seriously, although she didn't cry. Then I spanked girl #1. I thought I was spanking rather hard, for just using my bare hand. She said, "Ow!" and looked at me and burst into belly shaking laughter. I'm not talking trying-to-be-disrespectful-ha-ha-on-mom laughter. I'm talking hilarious laughter. What could I do but join in? We had a great laugh...and I'm still mystified.What would you do???