Answer
Your instructor pumped a lot of air out of the (very thick-walled) flask before handing it over.
A partial vacuum can drop the boiling point for water to well below $100^{\circ}C$. The instructor pumped just enough air out so that the water was just short of boiling (i.e., the boiling point was just a hair above room temperature). The heat from your hand is sufficient to boil the water under this reduced pressure.
Work Step by Step
This is discussed on page 327 (refer to Figure 17.14).
