Riddle

I climb higher as I get hotter. I can never escape from my crystal cage. What am I?

Choose an answer

A.Water
B.Alcohol
C.Steam
D.Mercury
Hint: It's a word with 7 letters, starting with "M".

Answer

Mercury

Why this is the answer

Liquid mercury inside a glass thermometer rises up the tube as the temperature increases, but it's always sealed inside the glass "cage."

This riddle is filed under "What Am I?" Riddles and rated Hard.