Final answer:
A body has gained heat when it absorbs thermal energy, which is usually evidenced by a rise in temperature. This is influenced by the substance's specific heat and overall heat capacity. However, a temperature increase can also occur without heat transfer through work done on a system, as well as remain constant during phase changes.
Step-by-step explanation:
We say a body has gained heat when it absorbs thermal energy, resulting in an increase in its internal energy and often in its temperature. Temperature change is a common indicator of heat transfer; if the temperature of a substance rises, it usually means that the substance has absorbed heat. This is dependent on the substance's specific heat, which is the amount of heat per unit mass required to raise the temperature by one degree Celsius. For example, when your body temperature rises by 1.00°C, it suggests that there has been a net gain of heat, assuming all other forms of heat transfer are balanced.
The heat capacity (C) of a substance is another concept pertinent to this discussion; it is the total quantity of heat a body absorbs or releases for every 1 degree Celsius change in temperature. This is why activities like exercise, which increase your body's temperature, are consistent with the experience of gaining heat.
Adding heat might not always result in a temperature rise. During phase changes, like the melting of ice, heat is absorbed but the temperature remains constant. Similarly, work done on a system, as demonstrated by Joule's experiment of stirring water, can increase the system's temperature without necessarily adding heat externally.
In the context of health, when our bodies become too warm, mechanisms such as sweating and increased blood circulation are activated to dissipate the excess heat, typically when a person is in a hot environment like a 40.0°C hot tub.