Given the widespread use of memory-safe languages, stu-dents must understand garbage collection well. Following a constructivist philosophy, an effective approach would be to have them implement garbage collectors. Unfortunately, a full implementation depends on substantial knowledge of compilers and runtime systems, which many courses do not cover or cannot assume. This paper presents an instructive approach to teaching gc, where students implement it atop a simplified stack and heap. Our approach eliminates enormous curricular depen-dencies while preserving the essence of gc algorithms. We take pains to enable testability, comprehensibility, and fa-cilitates debugging. Our approach has been successfully classroom-tested for several years at several institutions.