President Obama looks out the window of the Robben Island prison cell that once held Nelson Mandela. The president and his family visited the prison on Sunday. Gary Cameron /Reuters /Landov hide caption

itoggle caption Gary Cameron /Reuters /Landov

Friday in Pretoria, South Africa, people gathered outside a hospital to pray for former President Nelson Mandela. He remains in critical condition with a lung infection. Jeff J. Mitchell/Getty Images hide caption

itoggle caption Jeff J. Mitchell/Getty Images

Children release 95 balloons to mark Nelson Mandela's upcoming birthday outside the Mediclinic Heart Hospital where Mandela is being treated for a recurring lung infection in Pretoria, South Africa. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

A message for "Dear Tata" (Nelson Mandela) outside the hospital in Pretoria where he is being treated. Alexander Joe /AFP/Getty Images hide caption

itoggle caption Alexander Joe /AFP/Getty Images

Prayers for Nelson Mandela: At Regina Mundi church in Soweto township, South Africa, on Sunday, congregants prayed for the former South African leader. Dai Kurokawa /EPA /LANDOV hide caption

itoggle caption Dai Kurokawa /EPA /LANDOV