I am trying to come up with a suitable riddle for a simple door puzzle.

The door is a circular stone door with a carving of a man drinking, which is angled so his empty cup is upside down. The players need to fill it with steam to progress (the room has what they need to do this).

I'm struggling to find a riddle which solves to "fill my cup with steam". I'm drawing a total blank! Any suggestions gratefully received!

So far my best offering is:

My Survival’s Elementary

The Earth I Eat, The Air I breathe

So kindly fill my cup for me

which I feel is lame and likely to result in failure. the best clue is "elementary" meaning elements, so the last 2 elements are fire + water = steam.

Please, someone do better!

The one thing that I require,
Is in the beginning and ending of stars,
Followed by the whole of a soothing herbal remedy,
And lastly, the center of infirm scars,
Together combined to fill my cup.

*I know it's not very good, I'm better at solving riddles then I am at making them.*

Sorry if this isn't what you want I was just trying to help with giving a riddle that can help you out for your campaign.

An old man drink spoiled 

Drink air that is easy to see 

fill up a friend's cup to see the key.

Burning air,

Smoke of the sea,

With water which does not flow,

Fill my cup for me

anything lol?

What if it just bore the legend: 

"Fill my cup with water" (or "fill an old man's cup, for he is thirsty" if you wanna make it sound more personal)?

I think all the riddle you need is simply to present the players with the conundrum of how to fill an inverted cup, then let them try and problem-solve around it and see what solution they come up with.

Logically they might think of using steam, or they might use some water-shaping spell, which you as the DM are free to decide if their use of limited resources earns them a pass or if they have to keep trying until they arrive at the right answer. 

One thing to consider though, anytime you put a riddle on a door, is that this shouldn't be an essential door. There should be some extra treasure or reward behind it, that way they can still fail without disputing the game. 

Have you thought about placing steam related iconography around the room? Maybe a picture of a hot spring or lava when it touches water -- letting the players deduct from multiple elements within the room what the answer may be as opposed to a singular riddle. That way the riddle could be as easy as Red solo cup, fill me up.

I might toss the following verses under some weird pictures or carvings around the room:

My TEAMS pull a wagon, but this lifts my flagon.

My MATES sail on ships, but this brings drink to my lips.

Song TAMES the beast filled with malice, but this raises my chalice.  

The MEATS fill me up, but this hefts my cup.

Hopefully they can see that each of the italicized words are anagrams of STEAM.

Steam goes up so it would be hard to fill a cup with it

The cup is upside down, as it says in OP

Oh okay 

Quote from hobosomers >>

I might toss the following verses under some weird pictures or carvings around the room:

My TEAMS pull a wagon, but this lifts my flagon.

My MATES sail on ships, but this brings drink to my lips.

Song TAMES the beast filled with malice, but this raises my chalice.  

The MEATS fill me up, but this hefts my cup.

Hopefully they can see that each of the italicized words are anagrams of STEAM.

^ This. Hobosomers, I'm voting for you having the best riddle. I feel like anagrams are underused in riddles in general, let alone D&D.

Thank you all for the help here! I jut got in from work, am utterly exhausted, and collapsed on the bed for a nap. Naturally, my mind chose that particular moment to come up with the perfect riddle :D

The door remains the same, an upside down goblet and an upside down person holding it, as if drinking. Above the door is carved:

"One change of mates would be reversed

The last of teams should come in first

to fill my cup and quench my thirst."

I think it's suitably cryptic, and there's some good stuff behind here so I don't want it too easy (the first half of the cave system has been designed as full of steam so it's going to be somewhat on their minds!). I really like how this reads in the end, I'd never have thought about using anagrams!

Unsure of whether to capitalise the first letters in Mates and Teams, it might make it too obvious...

I'm reminded of the song lyrics from late in the last millennium: 

"I had some dreams

they were clouds in my coffee

Clouds in my coffee, " 

Work with the concept of capturing clouds in cups.

Quote from Ted_Ursi >>

I'm reminded of the song lyrics from late in the last millennium: 

"I had some dreams

they were clowns in my coffee

Clowns in my coffee, " 

Work with the concept of capturing clouds in cups.

 Corrected for how I hear the lyrics XD

Carly Simon, who is still alive BTW, probably cringes every time somebody does that.

"My cup runneth over, not under." (If the steam overflows out of the cup, it will flow up and "over".)

"Ever filling, never spilling."

"Not all fluids flow. Not all matter falls. When I get too heated, I rise above it all."