Conditions for crumpling: thin elastic manifold
Metric: preferred distance between all pairs of points
Elasticity: energy cost for altering this distance. 
Thin manifold: d-dimensional set of points much smaller in one direction than the others
Energy  =  
Bending  B
Stretching  S
1
g
R
R
Energy cost simplifies when local deformation is smooth
curvature
strain
Smooth: g << 1;  R >> h
h