The incongruity here is between Professor McGonagall with her hair in a tight bun -- a sign of extreme properness -- and her suggestion of 'letting their hair down'. In English, 'let one's hair down' means 'to cast off usual restraints, relax, and have fun'. This is why Rowling ...
A pair of new silkworm variety "Nongke No.2"(Hua Qiu × Chunri Yi. J9) with good comprehensive economic characters has been bred by the Silkworm Research Institute, Zhejiang Academy of Agricultural Sciences. It is a fourway crossed hybrid with double sex-limited larval markers for easier hybr...