Why is a frog stiff and not limp when dead?

When a frog dies, it undergoes a process called rigor mortis, where the muscles stiffen due to biochemical changes. After death, ATP (adenosine triphosphate) production ceases, preventing the muscles from relaxing. As calcium ions leak out of the cells, they cause the muscle fibers to contract and remain in that contracted state, leading to the stiffening. This rigidity typically starts a few hours after death and can last for several days before the muscles begin to decompose.