One question that's commonly asked is, wouldn't 2015 Jennifer remember seeing his younger self from 1985. First of all, I've introduced something called "temporal amnesia" in Time Switched and Ripple to the Future - where, if the past self travels into the future, the future self will temporarily have no memories of his younger self's actions in the future.

However, I'm not going into that for this topic, for one simple reason - I don't think 2015 Marty and Jennifer ever were the Marty and Jennifer from 1985 that we follow.

I do have major problems with the whole "Marty and Jennifer older selves exist in 2015, because the younger ones are destined to return to 1985" theory.

1. First and foremost, and I really wish more people would take note of this, if the Marty and Jennifer that we see in 2015 are the future versions of our Marty and Jennifer - then why on earth do they still lead unsuccessful lives in 2015?! This one bothers me the most.

2. Also, this timeline obviously doesn't take into account Biff stealing the almanac, and taking it back to 1955 - leading to the following adventures.

3. Also, what's to prevent Marty and Jennifer from suddenly deciding to settle in 2015. Yeah, I know Doc would tell them they can't - but, just for the sake of this scenario, Marty and Jennifer rebel against Doc's wishes. This is where I like the 24-hour ripple effect that Mike Mahoney came up with.

4. Then there's also the part where your own future isn't written, yet - so why would that part (Marty and Jennifer returning to 1985) be written?

So what about Old Biff remembering the flying DeLoean from 30 years ago? Well, I've always had a problem with that part. This might seem a bit convoluted, but the best I came up with is this. Old Biff remembering the flying DeLorean is a result of the timelines converging.

I think that it's just the future they will have if they keep their lives going the way they are.

I'm going with Mike Mahoney's 24-hour ripple effect.

Right, I agree, too! I don't agree with all of Mike Mahoney's theories, although one might feel that my idea of using a witch to alter history is just as unrealistic as his idea of a memory changing device - but the process is more introcate. The idea of such a device ever being invented just gives me the creeps, because just imagine if it ever fell into the wrong hands. The gravestone one doesn't quite set right with me, either - even though I can see where it would simply things a little.

The 24-hour ripple effect is the one that I'm in complete agreement with, though. It makes the most sense to me, when you consider the other options.