From my three y/o cousin
Our grandpa has problems with his ears and over the summer went almost entirely deaf. My cousin was a bit put out because he fell asleep in the lounge, and then wouldn't wake up no matter how many times she said, "Granpy? Granpyyy?"

She enlisted the help of babysitting Sivvus, who absentmindedly said, "Just poke him, he just can't hear you." This worked.

A few days later, when her mum came to pick her up, she poked her, grinned evilly and recited what she'd learned over her holiday: "It's rude to poke people but it's okay to poke old people because they're deaf."
(Sivvus trolley collision in the fruit aisle led to a jam, Fri 25 Nov 2011, 9:43, Reply)