Two men with Alzheimer's are sat in a park...

... when they hear an ice-cream van pull up nearby. Bob turns to Bill and asks 'do you want an ice-cream Bill?'
Bob says 'yes please, but don't forget the chocolate sauce.'

Bob says 'I won't forget, don't worry. Anything else?'

Bill says 'in that case, I'll have some chopped nuts on it too. Don't forget now.'

Bob says 'I won't, don't worry. Chocolate sauce and chopped nuts, coming right up' and

Bob wanders off in the direction of the ice-cream van.

After 40 minutes, Bob finally turns up with two hot-dogs. Bill says 'you fool Bob! I knew you'd forget! I wanted mustard on mine!'