Non-Newtonian substance - Tahini
Yes, tahini (and cornstarch-water mixtures) are dilatant, non-Newtonian fluids, with viscosity increasing with shear strain. See http://en.wikipedia.org/wiki/Dilatant for definition and https://www.youtube.com/watch?v=D-wxnID2q4A, https://www.youtube.com/watch?v=BleCJJAKkgw for demo.

Tahini is much better in hummus than in a swimming pool, though. ;-)