Transitional epithelium
Transitional epithelium is a tissue made of many layers of epithelial cells that can expand when filled with fluid or contract when there's a lack of fluid. It is thus 'transitional' since it does not have a fixed shape of form. When the bladder is full, the transitional epithelium lining it expands to contain urine. Whereas, when the bladder is empty, the transitional epithelium lining contracts and flattens in shape.