It may sound like something used by one of Harry Potter's wayward classmates at Hogwarts, but "invisible ink" has now been revealed by the university ombudsman as the latest university exam scam.
According to the latest annual report by the Office of the Independent Adjudicator (OIA) in Britain, a law student was caught red-handed with 24 pages of "unauthorised notes" written in invisible UV ink.
It is thought that the student managed to smuggle a UV light into the exam in order to decipher her invisible notes.
The student had made the invisible notes in her law statute book which she had bought into the exam hall with her, but was caught after being spotted looking at the notes by the invigilator.
The details of the "invisible ink" incident were disclosed in the report, which cited it as a case study of a student who had complained unsuccessfully about the penalty imposed on her for cheating.
It was one of 66 complaints on "academic misconduct, plagiarism and cheating" that were escalated to the OIA last year. Students can appeal to the OIA to review complaints against their university.
The use of invisible ink is the latest instance of students using technology to cheat in exams. Last month MPs and university chiefs called for "intrusive" airport-style searches after The Telegraph revealed that a growing number of students are sneaking tiny in-ear devices into exams that can whisper answers into their ears.
Official data has revealed for the first time that "cheat tech" is on the rise, as hundreds of students have been caught with covert technological devices during tests.
Lord Storey, who has campaigned to ban the rapidly growing industry of professional essay-writing services, said that the use of invisible ink in exams is "very ingenious".
He said that universities are aware that cheating is a "growing problem" as it affects the credibility of the institution. Lord Storey, who is the Liberal Democrats education spokesman for the House of Lords, added said that students were sometimes "desperate" to do well in exams which can lead them to searching for cheating technology online.
"The number of students is increasing all the time, particularly overseas students," he said. "Sometimes rich parents are paying the fees and the young person is desperate to do well in exams and make sure money wasn't wasted".
Invisible ink sets are available to buy for as little as £2 on internet shopping sites and are often marketed as children's toys. They can be useful for marking expensive possessions so that they are easier to detect in case they are stolen and sold on.
The ink itself includes phosphors, which emit light when exposed to radiation such as UV light.
Invisible ink has been reportedly used for a variety of crimes in the past, including by gangs passing secret messages to each other.
The al-Qaeda plotter Habib Ahmed, 32, was jailed in 2008 after being caught smuggling code books written in invisible ink into the country.
He was part of a British terror cell that police believe were planning a massacre in Britain.
A spokesman for Universities UK said: "Universities take cheating extremely seriously and have severe penalties for students found to be cheating.
"Academic misconduct is a breach of an institution's disciplinary regulations and can result in students being expelled from the university. Universities have become more experienced in detecting and dealing with all forms of cheating."