diff --git a/contents/algorithms/oop.md b/contents/algorithms/oop.md
index 2117786a..15883389 100644
--- a/contents/algorithms/oop.md
+++ b/contents/algorithms/oop.md
@@ -10,3 +10,4 @@ title: Object-Oriented Programming
 - Explain how you would design a HTTP server? Give examples of classes, methods, and interfaces. What are the challenges here?
 - Discuss algorithms and data structures for a garbage collector?
 - How would you implement an HR system to keep track of employee salaries and benefits?
+- How would you implement an Elevator system?