We say come to think of it when we are reminded of something--a fact or something that we have to do. If a coworker mentions that he's going downtown and this reminds you that you also need to go downtown to pick something up at a store, you can say, "Come to think of it, I do need to head down that direction. Do you mind if I come along?" Sometimes we use the phrase, "which reminds me" to mean the same thing.
Come to think of it is also used when we are reminded of a fact that is contrary to what we originally thought. For example, you're telling someone about a store, which you think is on Spring St., but in fact, it's not on Spring St., but a block from it. If someone challenges you because this person knows for a fact that the store is not on Spring St., and you realize your mistake, you can then say, "Actually, come to think of it, you're absolutely right. It isn't on Spring St. It's on Prince, like you said."
A: Did you hear about what Sam did at the party last night? He got up on the table and started yelling at people. He was so wasted.
B: Wait a second. Sam wasn't there last night. I was at the party all night.
A: Oh. Actually, you're right. Come to think of it, I saw Sam at another party the other night.
B: How much did you have to drink last night?
A: Not much. I was just a little confused. That's all.
B: Are you sure? Come to think of it, I think I saw you dancing on the table.
A: Ha. Ha. very funny.
contrary - the opposite
wasted - very drunk (in this context)
Ok folks, I hope you get to use this expression in the next few days. You can use it to make an excuse. "Come to think of it, I do need to get home early. I have a lot of homework to do."