I have a small cousin and she enjoys when I pick her up, which I can do pretty easily.

Sometimes though she decides she wants to make my life difficult, and when she decides so, she tells me she is "being heavy" and I can actually feel a substantial difference when I lift her, until her feet are not in contact with the ground anymore.

Now, assuming that this is not just an impression, what could be the cause of this? I assume that it is about how the force exerted by my hands is transmitted to her body.. or maybe usually she "helps" with her hands when I take her (even though I don't think so) and when she is being heavy she is just laying down.. 

What do you guys think?