When empty, the bladder is much smaller than when it is full. since the bladder must be able to stretch and expand as it fills, it makes sense that its inner lining is made of epithelium?

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.