# Back to the Future

I slip through your fingers as sand in a sieve.
A brief moment, an eternity, is when I live.

I'm nothing at all, yet I contain everything.
Time travellers leave me, but I'm always waiting.

I'm always moving, yet you're always within me.
Held between mighty ones who can never win me.

Inspired by Joe-You-Know's nice riddle, and by XKCD via the current Fortnightly Topic Challenge.

• I really like the wording of this riddle. Commented Jun 5, 2018 at 15:11

I think you are

The present

I slip through your fingers as sand in a sieve.

The present moment is always fleeting, as sand in a sieve.

A brief moment,

The present lasts no more than a moment.

an eternity, is when I live.

But it is always the present.

I'm nothing at all,

The present doesn't really have a duration, it's length is essentially zero.

yet I contain everything.

Everything is contained in the present.

Time travellers leave me,

Time travellers leave the present to move into the future, or past.

but I'm always waiting.

I guess when a time traveller moves, they will move to a new present time, from their perspective.

I'm always moving,

The present is ever-changing

yet you're always within me.

You are always within the present moment.

Held between mighty ones who can never win me.

The past and the future.

• I was going to write this, beaten by a few seconds :\ +1 Commented Jun 5, 2018 at 15:05
• Well, I was going for easy, but maybe it ended up a bit too easy ... Commented Jun 5, 2018 at 15:07
• @Randal'Thor Well, a riddle like this is good from time to time. Commented Jun 5, 2018 at 16:07
• Were I to look at this puzzle from the point of view of the special theory of relativity, I might have some qualms about both the lines in the second stanza :-)
– Bass
Commented Jun 8, 2018 at 7:02

EDIT: Sniped by hexomino!

Are you:

The Present?

I slip through your fingers as sand in a sieve.

The moment of the present is always moving, and we are always moving through time

A brief moment, an eternity, is when I live.

It's but an instant, but we always exist in it

I'm nothing at all, yet I contain everything.

It can be defined as infinitely small amount of time, but everything is always experiencing 'now'

Time travellers leave me, but I'm always waiting.

Time travelers leave the present, but wherever/whenever they arrive will become their 'present'

I'm always moving, yet you're always within me.

The present changes, but our existence moves along with it

Held between mighty ones who can never win me.

Referring to the past and the future, much longer periods of time that we never get to exist in

You are

Love

I slip through your fingers as sand in a sieve.

Love is hard to hold on to, especially if you're pining for someone you can't have

A brief moment, an eternity, is when I live.

Love can last the briefest moments or forever.

I'm nothing at all, yet I contain everything.

Love is everything, yet it's not physical or tangible

Time travellers leave me, but I'm always waiting.

I believe this is a Doctor Who reference where the Doctor leaves Rose trapped in another dimension

I'm always moving, yet you're always within me.

Love is fluid, but is always with you.

Held between mighty ones who can never win me.

Reference to Helen of Troy, who was fought over but never truly won over.

• Wow. That fits surprisingly well, apart from the last line. Commented Jun 5, 2018 at 15:07
• I didn't see the other answers until after I posted. Commented Jun 5, 2018 at 15:07

While I think other answers fit better,

You are

Memories

I slip through your fingers as sand in a sieve.

Created memories are forgotten in time

A brief moment, an eternity, is when I live.

Memories are created in an instant, and live on within us

I'm nothing at all, yet I contain everything.

We humans are a summation of memories, despite not being physically tangible (yet)

Time travellers leave me, but I'm always waiting.

Memories persist through time with the traveler, and the state of the universe with it, and can be re-implemented (think Back to the Future, specifically when Marty and Doc end up in the alternate timeline where Biff is successful; they set things back to a more favorable state)

I'm always moving, yet you're always within me.

Memories include the state of the remember-er; e.g. when you remember a childhood birthday party, you are as you were then, not as you are now.

Held between mighty ones who can never win me.

Not sure how 'Memories' could fit this. Perhaps even those with power and might cannot take your memories?