The most prestigious universities generally attract excellent scientists and have fine science programs. University campuses are usually open to the public, though permission from guards is sometimes required, and there may be some café or cafeteria or mensa or restaurant or even a university shop on site. Universities usually offer public lectures about ongoing research. Otherwise, their seminars and buildings are reserved for the students and the working faculty including post-doctoral researchers or professors. On weekends or holidays, many universities require special permits to enter. Universities compete on a worldwide basis; hence, they are not ordered by geographical position or alphabetized. Below is a list of the 20 highest-ranked universities according to 2013/2014 QS world university ranking (of course rankings may differ according to year and specific subject).