Here's why:

* Thermal Conductivity: Styrofoam is a much better insulator than paper. This means it conducts heat much slower. Insulators trap heat, while conductors allow heat to pass through easily.

* Air Pockets: Styrofoam is filled with tiny air pockets, which act as excellent insulators. Air itself is a poor conductor of heat, so these pockets slow down heat transfer.

* Density: Styrofoam is less dense than paper, which means it has more air pockets per unit volume. This contributes to its superior insulation properties.

In summary: Styrofoam's superior insulating properties due to its lower thermal conductivity and air pockets make it a better material for keeping water hot than paper.