Collision Detection

If you want to prevent parts or assemblies from overlapping when you move them, turn on collision detection in the Property Editor.

Parts can overlap in space. When you move them or change their shape with modeling tools, they are allowed to pass through each other.

Select the File menu and click the Preferences button to open the Preferences window.
Select Collision Detection from the category list on the left.
For Check for collisions between, specify whether to check for collisions between All Parts in the model, or Only Parts with Detect Collisions enabled in the Property Editor.

Tip:

Checking for many parts can significantly increase the time to detect collisions, so the All Parts option should be used judiciously. Select None to turn off collision detection entirely.
Enabling Detect Collisions will only prevent collisions when moving parts or assemblies. It will not prevent collisions that might occur due to changes in the geometry such as push/pulling faces, modifying dimensions, or maintaining a snap reference.
If you attempt to collide two parts by moving one of them, the part that is moving will stop at or before the point of collision. The moving part stops before the actual collision if grid snaps are on, allowing the part to move only in discrete increments. The part will stop at the last non-penetrating snap position. If you attempt to drag a part past this position, the part remains in the non-penetrating position and is highlighted to indicate that a collision would occur if it moved farther. The part that is blocking the motion is highlighted in red. If you continue to drag a part all the way through another part, the dragged part will move again once you have pulled it past the blocking part.