The Solid Command
The solid makes an object either solid or unsolid. All objects are solid by default. When an object is unsolid users cannot collide with it. However, it will still trigger a bump if users pass through it.
The optional name argument specifies the name of the object to be made solid or unsolid. Object names are assigned via the name command. If omitted, the solid command applies to this object.
The flag argument is required and specifies whether the object should be made solid or unsolid. flag can be either "on", "true", or "yes" to make an object solid, and either "off", "false", or "no" to make an object unsolid.
This makes an object which is unsolid.
This creates an object which, when clicked on, makes the object named "trapdoor" unsolid. For this behavior to work the "trapdoor" object would need to name itself with the action: