Mirror modifier

Steps

1. Add a UV Sphere
   Shift + A → Mesh → UV Sphere
   
   

2. Delete half of the sphere
   
   

   • Go to Edit Mode
     
     

   • Select half of the sphere (e.g., use Wireframe view Z → Wireframe, then box-select)
     
     

   • Press X → Faces
     
     

3. Add a Mirror Modifier
   
   

   • Go to Modifier Tab (wrench icon)
     
     

   • Add Mirror
     
     

   • Choose the axis (usually X)
     
     

   • Turn on Clipping
     
     

✅ Now when you edit one side, the other side updates automatically.