Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Just a few days ago, Beth waded into her family's pond. Now when she stepped off the bank the pond was hard and she could walk on the surface. What happened?
a cold front had moved into the area
The pond froze.