Physical Unclonable Functions (PUFs) extract unique chip signatures from process variations. They are used in identification, authentication, integrity verification, and anti-counterfeiting tasks. We introduce new PUF techniques that extract bits from pairwise skews between sinks of a clock network. These techniques inherit the stability of clock network, but require a return network to deliver clock pulses to a certain region, where they are compared. Our algorithms select equidistant sinks and route the return network, then derive chip-specific random bits from available data with a moderate overhead. SPICE-based evaluation of clock-PUFs using a 45nm CMOS technology validates the operability, stability, uniqueness, randomness, and their low overhead.