In this task, you're given an article and an answer. Your task is to generate the fill-in-the-blank question for this answer based on the given article. The blanks are always at the end of the question, and can be filled with more than one word or phrase.
Q: Article: Each student at an engineering college in Canada gets a steel ring upon graduation in addition to graduation certificate. There is a story behind the unusual ring. Several years ago, a graduate from this engineering college designed an iron-steel bridge. Unfortunately, because of many mistakes in his design, the bridge collapsed soon after it came into use. This event shook the engineering college. And the college authorities decided to take back the framework of the iron-steel bridge and use it as raw material for making rings to give the students upon their graduation. The college wants the students to remember this lesson forever so as to avoid making such mistakes again.
Answer: a warning not to make iron-steel bridge
A:
Upon graduation each student is given a steel ring as _ .