Star came into the bathroom, where she saw Marco brushing his teeth with a red toothbrush. "Marco, why are you using my toothbrush?"
This got the boy's attention, who stopped what he was doing and answered, "What are you talking about? This is my toothbrush."
"How can you tell? That toothbrush is red!"
"My toothbrush is red, Star."
"...Wait, if that toothbrush is yours, and we've both been using it..."