Any kid will run any errand for you if you ask at bedtime.

The joke hinges on a familiar parental tactic: children are most compliant when they are already in bed and want to avoid getting up. By asking at bedtime, the adult exploits the child’s desire to stay comfortable and end the interaction quickly—so the child agrees to “any errand” in principle, even if they would resist during the day. The humor comes from the exaggeration (“any errand”) and the sly acknowledgment of minor manipulation in family life. It also gently satirizes how timing and context can matter more than the request itself in getting cooperation.