Weight painted mesh stays in place when rotated I’m new to Blender and trying to rig a head but the head and body are one mesh so I used weight painting to make sure the head bone only controlled the head. When I rotate the head bone the head deforms, with part of it staying in place rather than moving as one unit. The entire head is uniformly weight painted so I’m not sure why this happens. I also checked to make sure none of the other bones have weight painted control over this area in the head.

I am unable to add an attachment so I have a video link of what is happening here: https://imgur.com/a/hJbVS41

Also as a side note I have no idea why the hands move when I move the head (there is no weight paint there).

Any help with these two issues would be greatly appreciated.
Select your head (or weighted object). Go to weight paint mode. Open in the object's properties your vertex group, there you should see a list of all your bones organized as vertex group's influences. Use your keyboard's down arrow to go from top to bottom, one by one until you see the head bone (it should color your head in red color: full influence), keep going down the list and there should be other vertex group influenciating your head. If so: stay on that vertex group,now switch your painting weight brush from DRAW to subtract. Strength 1, weight 1. "Paint to delete" that influence. Repeat for all remaining groups that have influence. Go back to head bone vertex group, switch back the weight brush to DRAW. Strenght to 1, weight to 1. Move your bone head until your head deforms, now paint the remaining vertices (left behind) and they will sit in place now.