5.15 save matrix to file, and read it again

SetDirectory[NotebookDirectory[]]; 
list = {{3, 4, 5}, {4, 5, 6}}; 
Export["data.txt", list]
 

To read it later, say after closing and restarting Mathematica again to continue working on the data

SetDirectory[NotebookDirectory[]]; 
list = ToExpression@Import["data.txt", "List"] 
 
    {{3, 4, 5}, {4, 5, 6}}