Liberia has been named number one in the world when it comes to helping strangers, according to the World Giving Index, an annual ranking of people's generosity.Helping strangers is one of the three criteria the UK-based Charities Aid Foundation used to draw up the overall index of generosity.It also looked at how much money people donate to charity and how much time people give to volunteering.The index was based on a 10-year study that surveyed 1.3 million people across the globe.In the overall rankings, Kenya was listed as the most generous country in Africa and the 11th most generous in the world.Liberia was listed as 17th, Sierra Leone 20th and Nigeria 22nd.But it is in helping strangers that Africa excelled.Including Liberia, there were seven African countries in the top 10 in that category: Sierra Leone (second), Kenya (fourth), Zambia (fifth), Uganda (sixth), Nigeria (seventh) and Malawi (joint 10th).