Squash and stretch is considered the most important of Disney's 12 principles because it solves animation's fundamental problem: making rigid objects feel alive. Developed in the 1930s at Disney, it emerged from observing how real flesh and rubber deform under force while maintaining constant volume.

Volume Preservation: When an object squashes, it must widen. When it stretches, it must narrow. This constraint creates believability—violate it and objects appear to grow or shrink rather than deform.

Force Visualization: Squash and stretch makes invisible forces visible. A ball squashing on impact shows us the floor's resistance. A character stretching mid-leap reveals velocity and momentum.

Not all objects deform equally:

Even "rigid" objects benefit from 1-2% deformation—it prevents the dead, mechanical feel.

Timing amplifies squash/stretch: Fast impacts demand more squash; slow floats need gentle stretch. The deformation amount must match velocity.

Anticipation uses stretch: A character winding up for a jump often stretches slightly before the motion begins.

Follow-through extends it: After landing, the squash ripples through secondary elements (hair, clothing, flesh).

Start with 10% deformation for energetic motions, 2-3% for subtle polish. Adjust based on material and tone. When in doubt, less is more—squash/stretch should be felt, not consciously noticed.