Nelson Mandela Nelson Mandela

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

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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

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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

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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

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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

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Dai Kurokawa /EPA /LANDOV