In doom level-editing terms, THINGs are monsters, weapons, ammo, obstacles, items, keys, player start positions and teleport destinations; in short, everything in the doom world that is not a wall, floor or ceiling.
Use the Thing Mode to do the following:
(a) Create, delete and reposition things.
(b) Change the types and properties of things.
You enter Thing Mode by choosing Thing from the Mode menu, or using its keyboard equivalent (command-T).
While in thing mode, vertices are not displayed. Linedefs and sectors are displayed to provide reference points, but they cannot be selected.
The manipulation of things is identical to the manipulation of vertices (see section above). Use a command-click to create a thing. Click on a thing to select it and elsewhere to deselect all selected things. Selected things are displayed as filled diamonds, while unselected things are displayed as hollow diamonds. Each general type of thing (e.g. monsters or keys) is represented by a different colour diamond in the map area. Things may be framed by a square or circle which represents their actual size when playing.
 
Use shift-clicks to add a thing to, or remove a thing from, a set already selected. Selected things can be dragged to new positions. Use the Clear or Command-Delete key(s) to delete all selected things.
A thing's type and its properties are set from the controls in the properties area. Use the popup menu to set its type. The three checkboxes labelled "1&2", "3" and "4&5" determine the skill levels at which the selected thing(s) will appear. The checkbox labelled "Deaf" is only useful for monsters, and determines whether a monster will wake up when it hears a noise. The box labelled "Multi" determines whether an item will appear in multiplayer games. Finally, the box labelled "Facing" gives the direction the monster initially faces. You can change the facing direction by clicking on the multiple arrows symbol.
 
When multiple things are selected, any changes to the properties will affect all selected things.
