The all-knowing toddler

How can it be that my baby can now walk, almost talk and understand everything? I say to him, 'find Mr Snuffy darling', and he wanders off and finds Mr Snuffy. I say to him, 'put the tissue in the rubbish bin', and he totters off into the kitchen, I hear the lid open and close, and he totters back victoriously. I can be talking to someone else about a jacket or a hat, and Harper will appear with his jacket and his hat, and then sit on the floor for me to put them on him. But why is it when I give him a glass of juice and ask him to sit at the table and hold the glass with both hands, it's time for some disco dancing?