2D character animation (flour sack)

I'm learning now principle of "bouncing ball" and
"squash and stretch" by using flour sack. In my mind,
To develop my 3D animation skills, it is really helpful to
learning traditional animation. 3D animator tend to rely
on key frame animation and afraid to construct the
animation by keying every frame. To develop certain
shot, sometimes (almost everytime) it is really
important to check every frame. Sounds it is super basic,
but worth reminding in your head again and again.