What's the best way to travel from Nepal to Tibet?

Though Bhutan's economy is one of the world's smallest, it has grown very rapidly with about 8% in 2005 and 14% in 2006. As of March 2006, Bhutan's per capita income was US$ 1,321 making it the highest in South Asia. Bhutan's standard of living grew and is one of the best performing in South Asia. Bhutan's economy is one of the world's smallest and least developed, and is based on agriculture, forestry, and the sale of hydroelectric power to India. Agriculture provides the main livelihood for more than 80% of the population. Agrarian practices consist largely of subsistence farming and animal husbandry. Handicrafts, particularly weaving and the manufacture of religious art for home altars are a small cottage industry and a source of income for some. A landscape that varies from hilly to ruggedly mountainous has made the building of roads, and other infrastructure, difficult and expensive. This, and a lack of access to the sea, has meant that Bhutan has never been able to benefit from significant trading of its produce. Bhutan currently does not have a railway system, though Indian Railways plans to link up southern Bhutan with its vast network under an agreement signed in January 2005. The historic trade routes over the high Himalayas, which connected India to Tibet, have been closed since the 1959 military takeover of Tibet (although smuggling activity still brings Chinese goods into Bhutan).