Why is sometimes more difficult to lift a baby? 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?
 A: You say that you "can actually feel a substantial difference when I lift her, until her feet are not in contact with the ground anymore". That makes perfect sense i.e. she is also participating in the lifting process by pushing off the ground except when she's being difficult.
Once she's off the ground, as long as you're holding her in the same way and she isn't making any other squirmy motion that makes it harder for you to maintain your grip, there is no difference.
A: From Making yourself heavier

Yes, it is possible to make yourself harder to lift. By shifting your center of gravity , which is usually referred to as your hips, down you can make it much harder to be lifted. It depends on which kind of lift, but this usually does the trick. Shifting your center of gravity does not make you heavier. It shifts the point where your body is balanced. 
  If someone tries to lift you up, try bending your knees. That shifts your center of balance so that you are harder to pick up.

If this idea is true, it is impressive how much intuitive physical knowledge we have at an early age.
