All living things require nutrition, or the intake and use of food. Energy is released from food by a process called respiration. As living things exhale they give off carbon dioxide and water. A special activity that removes wastes produced by living things is known as excretion. Living things react, or respond, to changes in their surroundings. People might react to cold weather by shivering, which helps to warm them. Living things also react to their surroundings by moving. Plants gradually bend toward sunlight. Another activity of living things is growth, or to increase in size or amount of material contained. Living things produce offspring through reproduction.