Juan boiled a liquid and recorded the temperature at 1-minute intervals until the liquid had nearly boiled away. how do you explain the shape of the curve he drew

Before I could be sure, I'd really need to see the curve that Juan drew.

My wild guess is that his graph was a straight horizontal line.
Once the whole pot of water gets up to boiling temperature,
adding heat doesn't change the temperature any more.  The
added heat is what changes some of the water to steam,
while the temperature stays constant..

The geological theory that states that pieces of earth's lithosphere are in constant slow motion is the theory of plate tectonics.

Athermometer to measure the temperature of course