I saw this paragraph at the Game Revolutions site.
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Another cool little quirk is the imaginary friend for a young child. Little Timmy can play with his buddy, who can be dressed up in all kinds of costumes, and you can issue suggestions to the imaginary friend as long as you’re looking at them through the kid. Click over to another Sim, however, and the friend disappears, while little Timmy just looks like he’s playing on the see-saw all by himself."