What do you need to add inside a box to strengthen its sides before packing a heavy item?

To enhance the structural integrity of a box, especially when packing heavy items, incorporating a liner is crucial. A liner, typically made from materials like cardboard or other sturdy materials, reinforces the walls of the box, allowing it to better withstand weight and stress during handling and transit. This added layer of support helps prevent buckling or collapsing, which can easily occur when a heavy item is placed within a less sturdy box.

While padding, support beams, and dividers serve important functions in packing — such as cushioning items, providing additional strength, or separating different contents within a box, respectively — they do not specifically address the need for reinforcing the sides of a box as effectively as a liner does. Therefore, using a liner specifically targets the enhancement of the box's ability to bear heavy loads, making it the most appropriate choice for this scenario.