How will you trace 9 edges of the cube without lifting the pen?

Answer 1

There are many ways: Here is one.

Imagine a unit cube in the 3-d Cartesian space and follow the sequence of vertices as given below.

Start at (0, 0, 0)

Go Right to (1, 0 ,0)

Go Up to (1, 1, 0)

Go Back to (1, 1, 1)

Go Down to (1, 0 1)

Go Left to (0, 0, 1)

Go Forward to (0, 0, 0)

Go Up to (0, 1, 0)

Go Back to (0, 1, 1)

Go Right to (1, 1, 1)

Incidentally, Back refers to moving along the Z axis - in the front-to-back direction, not necessarily returning.