This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.

Yale seniors at graduation smash clay pipes underfoot to symbolize passage from their "bright college years," though in recent history the pipes have been replaced with "bubble pipes". ("Bright College Years," the University's alma mater, was penned in 1881 by Henry Durand, Class of 1881, to the tune of Die Wacht am Rhein.) Yale's student tour guides tell visitors that students consider it good luck to rub the toe of the statue of Theodore Dwight Woolsey on Old Campus. Actual students rarely do so. In the second half of the 20th century Bladderball, a campus-wide game played with a large inflatable ball, became a popular tradition but was banned by administration due to safety concerns. In spite of administration opposition, students revived the game in 2009, 2011, and 2014, but its future remains uncertain.
What do seniors crush to celebrate graduation?