Mommy's sink isn't working, and she's been trying to fix it when you show up. You offer to give mommy a hand, getting down underneath the sink. She is trying to look at what you are doing when her hand brushes against your cock, and she feels it getting harder in her hand. You're being such a good son, helping mommy with her plumbing problems...she just wants to give you a nice reward for being such a good boy...