We review the weak-decay and spectroscopy properties of baryons with two charmed quarks. We also present the convergent speculations on exotic mesons (QQq̄q̄) with two heavy quarks and two light antiquarks.

A mostly expository account of old questions about the relationship between polyhedra and topological manifolds. Topics are old topological results, new gauge theory results (with speculations about next directions), and history of the questions. MR classifications 57Q15, 01A60, 57R58.