Here's why:

* Viscosity refers to a fluid's resistance to flow. High viscosity means a fluid flows slowly and is thick (like honey). Low viscosity means a fluid flows easily and is thin (like water).

* Shield volcanoes are formed by eruptions of very fluid, low-viscosity lava. This lava flows easily and spreads out over large distances, creating the characteristic wide, gently sloping shape of a shield volcano.

In contrast, stratovolcanoes are formed by eruptions of more viscous lava, which flows slowly and builds up steep, conical slopes.