Choose some to draw, mould or paint.
Mould a square prism of clay.
With thread, cut it into two equal parts like those in the picture. Each of these two equal parts is a right tri-prism.
Look at the three oblong faces of your right tri-prism. One face is wider than either of the others. When a right tri-prism is resting on its widest oblong face, it looks like a roof.
What does it look like in other positions?
Draw a circle. Fold and cut it into two equal parts. Each is a semicircle.
Place a hemisphere on a book. Raise the book to the level of your eyes. Draw the shape of the hemisphere. You have drawn a semicircle.