Lost of Appetite?

I was wondering why these 2 days Joel seems to have no appetite. He has not been able to finish his usual portion of rice. Usually he will ask for second helping. Well, I kinda got my answer this evening, before Joel's bath time. He stood in front of my mum's mirror, tip toe and said "Mummy, you know, if I have not been eating so much, i will be this tall already." I then wonder whether he is trying to lose some weight? In my heart i was thinking, yes, Joel is quite vain, but how can a 7 yrs old be so vain, that is how he can think of losing weight by eating less? So i asked, " you mean you purposely eat less so that you will grow taller?" Well, i have his reassurance. He said " No, I really feel full, that is why cannot finish the rice." What a relief. But knowing Joel (Joel is the sort of boy who may be young but know how to "manipulate" , to tell you what you want to hear), so I shall keep a watch on him to make sure