Building a sink
Create a cube. With STRETCH change its height at Z = 0.1m.
In SUBDIVIDE mode, divide the cube into 8 x 8 small regular cubes.
In mode MOVE, raise the cube to Z = 0.6 (you can type 0.6 in the Z box)
Go to CURB mode, with the sub-option "both dimensions".
Click on the cube exactly in the center of the grid and lower it to Z = 0.1 (you can type 0.1 in the Z box).
And here's your sink !
If you want a bathtub you can stretch it with STRETCH.