For things that are physically impossible, don't bother with dice. As DM, dice are for when the group needs to determine whether something happens that's probable but neither impossible nor guaranteed. For impossible things, you're allowed to just say so:


  Player: I fly to the moon!
  
  DM: …You can't. You don't have wings, a ship, a Instant Moon Travel power, or anything like that.
  
  Player: Can't I just roll against a really high DC?
  
  DM: No.


Perfectly acceptable.

When there isn't a rule allowing something to happen, it's the DM's prerogative to judge whether something is possible, and how difficult it is. Things that are very easy like "I walk up the stairs" don't need rolls for you to say "OK" to — neither do impossible things need rolls for you to say "No" to.

Nota bene, for truly "out of the ordinary" but not impossible things there is Difficulty Class and Damage by Level (DMG, p. 42), which gives you a framework for assigning DCs to tasks that aren't otherwise covered by another rule in the game. Punching down a whole masonry wall with bare fists shouldn't ever need a DC (because it's just not happening), but punching a hole in a wall while accidentally turned into a golem might call for a high DC, and that's where to find it.

What should a DM do if a player wants to do something impossible?
Im trying to export a model for the Euro Truck Simulator 2 game with the Blender2SCS addon and I get the following error, any help?

Traceback (most recent call last):
  File "C:\Program Files\Blender Foundation\Blender\2.72\scripts\addons\io_scene
_scs\__init__.py", line 1951, in execute
    error = export_scs.save(filepath, origin_path, root_object, self.copy_textur
es, int(self.pmg_version))
  File "C:\Program Files\Blender Foundation\Blender\2.72\scripts\addons\io_scene
_scs\export_scs.py", line 27, in save
    status, ob = export_pmd.save(exportpath, originpath, root_ob, copy_textures)

  File "C:\Program Files\Blender Foundation\Blender\2.72\scripts\addons\io_scene
_scs\export_pmd.py", line 305, in save
    pmd.write(f, exportpath, originpath, copy_tex)
  File "C:\Program Files\Blender Foundation\Blender\2.72\scripts\addons\io_scene
_scs\export_pmd.py", line 267, in write
    ret_ob = exp_mat.write(copy_tex)
  File "C:\Program Files\Blender Foundation\Blender\2.72\scripts\addons\io_scene
_scs\export_mat.py", line 91, in write
    for option in self.__options.keys():
AttributeError: 'str' object has no attribute 'keys'

location: &lt;unknown location&gt;:-1

location: &lt;unknown location&gt;:-1

