Nematic elastomers and glasses are solids that display spontaneous distortion
under external stimuli. Recent advances in the synthesis of sheets with
controlled heterogeneities have enabled their actuation into non-trivial shapes
with unprecedented energy density. Thus, these have emerged as powerful
candidates for soft actuators. To further this potential, we introduce the key
metric constraint which governs shape changing actuation in these sheets. We
then highlight the richness of shapes amenable to this constraint through two
broad classes of examples which we term nonisometric origami and lifted
surfaces. Finally, we comment on the derivation of the metric constraint, which
arises from energy minimization in the interplay of stretching, bending and
heterogeneity in these sheets
