If you recollect an experience...
If you doubt something that you've heard...
If you can't comprehend something...
If you consider someone to be attractive...
If an idea occurs to you...
If you contemplate your future...
If somebody reminds you of another person...
If you analyse a problem...
If you suspect something is true...
If you memorize a number...
you think it might be true but you aren't sure
you don't understand it.
you think about it for quite a long time
you think about it at a later date
you think it probably isn't true
you learn it so that you won't forget it.
it comes to your mind suddenly.
they make you think of that another person.
you think about it in a logical way.
that is your opinion