All planets are spherical, why are they not cubical or cylindrical?

The bulb of a thermometer is spherical and that of another is cylindrical Equal quantity of mercury is filled in them Then .

'A rifle bullet is cylindrical'. Why?

Two full tanks, one shaped like a cylinder and the other like a sphere, contain crude oil. The cylindrical tank holds 600 m3 more than the spherical tank. After 400 m3 of crude has been taken out from each tank the cylindrical tank contains twice the amount of oil than in the spherical tank. How many cubic meters of oil did the cylindrical tank have when it was full?

Why drops and bubbles are spherical in shape?