This is of course a news article about the findings of one study but the results are still interesting. It would be nice to see some more studies on this. Apparently babies as young as 6 months can tell if you're teasing them by with holding a toy.
http://www.ottawasun.com/news/canada/2010/02/08/12791656.html
http://www.ottawasun.com/news/canada/2010/02/08/12791656.html