Final answer:
Fire fighters may need to hold on to the self-expanding portable tank while filling it, as it can expand due to thermal properties. Tightly capped tanks may overflow or burst, so it's important to allow for expansion and contraction.
Step-by-step explanation:
Fire fighters may need to hold on to the self-expanding portable tank while filling it. This is because when the tank is filled with a gas or liquid, it expands due to thermal properties. If the tank is tightly capped, it can lead to overflow or bursting. To avoid this, the tank has air gaps which allow it to expand and contract without stressing the container.