How Do You Not Laugh?

Lately, Lucas has been sneaky. And while being deceitful is a concern in and of itself, he goes beyond that with the fibbing he does when caught. I'm beside myself. I've never had to deal with this, so, to be quite honest, I don't know what to do. Except, on occasion, to laugh.

Elsa was given a hip, girlie Birthday gift from a sweet friend of mine. The problem is that Lucas really likes it, too.

The other night as I was tucking the kids into bed, I noted multiple dark lines across Luke's denim-blue pillow case.

"Honey, what's all over your pillow case?"
"Um, it's just wet."
Feeling the pillow..."How did it get wet? Sweetie, it doesn't feel wet. What's on the pillow case?"
"Seriously, Mom, it's wet."
"Lucas, the pillowcase isn't wet. I'm touching it and it's not wet. Honey, I'm really more upset at this point about the truth not being told. Can you tell me what it is? Did you draw on the pillowcase?" ( A side note...Lucas has been graffiti-ing everything - including the neighborhood playground...so drawing on things is an issue for us.)
"Mom, if I tell you, please don't be upset with me."
"Tell me, please."
"It's Elsa's lip gloss."
(At this point, I'm thinking...we have a gazillion markers in the house and he used lip gloss...that's bizarre...maybe he thought it was a marker...no, he knows what lip gloss is because he tries to put it on..should I be worried?)
"LU-CAS (emphasis on both syllables), why on earth would you draw on the pillow case with lip gloss?"
"Well, the lip gloss is vanilla...and I wanted my pillow case to smell good."