Teachers were the most important factor in nation building, said President Pranab Mukherjee on Monday "For them sky is the limit. No civilisational progress could have been possible if teachers did not give their wisdom to society on the basis of which progress takes place."Our tradition is that of 'Guru-Shishya Parampara' and teachers are the most important factor in nation building," he said at the "In-Residence" programme for 12 award winning school teachers from states/union territories.