Can stones maintain a constant temperature?
No; they have no mechanism by which they regulate or maintain a specific temperature. They tend toward the ambient temperature of their surroundings.
Constant body temperature is maintained by warm blooded organisms - birds, mammals. The body temperature is maintained constant irrespective of an increase or decrease in temperature of the surrounding. The constant temperature is maintained by temperature haemostasis. This is achieved by altering the metabolic rate and rate of perspiration and urination. Maintaining constant temperature is also called as homeothermy.
This is a very broad question since there are many different types of homeostasis. It is possible to maintain a constant temperature, or a constant location, or a constant pH, or a constant velocity, or a constant luminosity, etc. etc. Without knowing what parameter you are concerned about, there is no point in describing a method of obtaining homeostasis.
As the temperature of a gas increases the volume of the gas must also increase to maintain constant pressure?
The gas volume become constant when the pressure is increased to a point that makes the distance between the gas molecules equal to zero at this point no more increase of temperature with pressure is observed. Or if the pressure and temperature are kept constant within a system then the volume can also be constant as long as you are able to maintain the pressure and temperature at constant level.
Which law when temperature of a gas increases the volume of the gas must also increase to maintain constant pressure?
The concept of increased temperature calling for an increase in volume to maintain constant pressure can be found in the combined and/or ideal gas law. The combined gas law is PV=kNT, where P is pressure, V is volume, k is Boltzman's constant, N is number of gas molecules and T is temperature. The ideal gas law is PV=nRT where P is pressure, V is volume, n is number of moles of gas, R is gas…
By the definitions of warm blooded and cold blooded, animals cannot be both. Cold blooded means that the animal cannot maintain a constant body temperature and instead its body temperature is dependent on the environment. Warm blooded refers to animals that can maintain a relatively constant body temperature independent of their environment.