Is it bad that my kid's lies make me laugh?

I don't know why but my kids loathe washing their hair.  Last night Ella was taking a shower while I quickly took Rand to Awanas.  I came home and asked if she had washed her hair yet.  She said yes, while I was gone taking Rand.  I looked at her mostly-dry hair and said, "Hmmmm.  I doubt that." 

She replied, "Oh, I forgot.  Daddy washed it." 

"Ok.  If I go and asked Daddy if he washed your hair, will he agree with you?"

Ella: "Probably not.  You can wash my hair, mom."