Tadpoles excrete ammonia but shift to urea production during metamorphosis. Blood urea and creatinine levels will be elevated. Heating of the solution generates hydroxide ions from the hydrolysis of urea. Excretion of urea occurs via the kidneys. After cooling, pure crystals of urea nitrate form. Dissolved in the water are trace amounts of minerals, lactic acid, and urea. Early stages in the reaction of formaldehyde and urea produce bis(hydroxymethyl)urea. Hydrolysis of the imine derives "N"-carbamoylputrescine followed with hydrolysis of the urea to form putrescine. There are two general methods for the measurement of urea nitrogen. It is produced from urea and formaldehyde. Glyoxylic acid gives heterocycles upon condensation with urea and 1,2-diaminobenzene. The excretion of urea is called ureotelism. Examples include fermentation of carbohydrates, short chain fatty acids, and urea cycling. Furthermore, they contain solute molecules like sugar and urea. A number of techniques, including fertilization by iron, urea and phosphorus have been proposed.