Lazy Evaluation
Q. What is lazy evaluation?
A. It's a policy of only evaluating forms whose values are needed by a consumer, such as a print request.
If Scheme fully supported lazy evaluation, then
(CAR (LIST 1 (+ 2 3) (* 4 5) (/ 6 7)))
would not result in any arithmetic actually being performed, since only the element 1 needs to be returned.