There is a difference between "think of" and "think about."
Some examples of "think of":
A: "What did you think of Mary Jones's speech?"
B: "I didn't think much of it."
A is asking B's opinion on Mary Jones's speech. B replies that she did not think it was a very good speech.
A: "Have you thought of packing water bottles?"
A is asking if it has occurred to B to pack water bottles.
"Think about" has a different meaning. It implies a longer period of contemplation on the matter at hand. Using the same examples, the phrase changes the meanings:
A: Did you think about Mary Jones's speech?"
B: No, I haven't thought much about it yet.
In this case, it is likely that A and B have previously discussed forming some kind of opinion on Mary Jones's speech. A is asking B if she has come to an opinion yet. B is replying that she has not had time to form an opinion yet.
Or else:
A: "Have you thought about packing water bottles?"
This question suggests that A and B had previously discussed the possibility of packing water bottles, and A is asking B if she has given the matter any more thought
