When do you use "ponder" and when do you use "mull over"?

Question

In the following sample sentences from the Oxford Advanced Learner's Dictionary, it seems to me that the two words are interchangeable in all instances without even the smallest change of nuance / connotation:

• "This was something I had been pondering about for some time." (Source)
• "I need some time to mull it over before making a decision." (Source)

Here are the definitions from the same dictionary, which are almost identical to each other:

• ponder: to think about something carefully for a period of time
• mull over: to spend time thinking carefully about a plan or proposal

When do you use "ponder" and when do you use "mull over"? Can I have a few scenarios that the choice makes a difference?

Answer 1

1. Note the Oxford definition that you ponder "something," but mull over a concrete "plan or proposal."

2. Ponder more abstract or creative thinking, and mull over more concrete decisions to make. Ponder some possibilities, then mull them over and decide in the morning.

3. Use ponder for divergent thinking, mull over for convergent thinking.