Scholarly pursuits gave way to a soda taste test, dismantling a car and other bizarre pursuits as California Institute of Technology underclassmen in Pasadena, Calif., tried to break into seniors' cleverly defended rooms.

In an annual ritual known as Ditch Day on Thursday, the students solved puzzles or met elaborate requirements to gain entry to the rooms of seniors, who remained on campus at the risk of being tied to a tree."We had a report that a senior came back and was tied to a tree," said Mary O'Brien, 19, of Houston, who fielded questions at "Ditch Day Central."

For obscure reasons the challenges left by the seniors are known as "stacks."

O'Brien said one stack he came across involved disassembling a car and making the pieces fit in four 55-gallon drums.

Students working on other stacks found they had to climb buildings.