Wall is a natural sign. Take your two open hands (with thumbs tucked in) and hold them out flat, side by side, palms facing out. Then slide them apart in opposite directions, as if to indicate the length or breadth of the wall you are indicating.
Teach your baby the sign for wall in conjunction with other parts of your house, and especially if your baby is riveted to the wall due to some bright colors or decor that catch their eye.
The sign for wall is easy to learn, particularly if you have attractive wall displays that capture the attention of your baby.