Task: Question Answering
Input (English): Passage: The ice must be cut through, broken up, or melted.  Tools can be directly pushed into snow and firn (snow that is compressed, but not yet turned to ice, which typically happens at a depth of 60 metres (200 ft) to 120 metres (390 ft)); this method is not effective in ice, but it is perfectly adequate for obtaining samples from the uppermost layers.  For ice, two options are percussion drilling and rotary drilling.  Percussion drilling uses a sharp tool such as a chisel, which strikes the ice to fracture and fragment it.  More common are rotary cutting tools, which have a rotating blade or set of blades at the bottom of the borehole to cut away the ice.  For small tools the rotation can be provided by hand, using a T-handle or a carpenter's brace.  Some tools can also be set up to make use of ordinary household power drills, or they may include a motor to drive the rotation.  If the torque is supplied from the surface, then the entire drill string must be rigid so that it can be rotated; but it is also possible to place a motor just above the bottom of the drill string, and have it supply power directly to the drill bit.If the ice is to be melted instead of cut, then heat must be generated.  An electrical heater built into the drill string can heat the ice directly, or it can heat the material it is embedded in, which in turn heats the ice.  Heat can also be sent down the drill string; hot water or steam pumped down from the surface can be used to heat a metal drillhead, or the water or steam can be allowed to emerge from the drillhead and melt the ice directly.  In at least one case a drilling project experimented with heating the drillhead on the surface, and then lowering it into the hole.Many ice drilling locations are very difficult to access, and drills must be designed so that they can be transported to the drill site.  The equipment should be as light and portable as possible.  It is helpful if the equipment can be broken down so that the individual components can be carried separately, thus reducing the burden for hand-carrying, if required.  Fuel, for steam or hot water drills, or for a generator to provide power, must also be transported, and this weight has to be taken into account as well. 
Question: What object's weight has to be taken into account as well?
Output (English): drills.