  Following Sullivan's spacial realization of a differential algebra, we
construct a universal integrating Lie 2-groupoid for every Lie algebroid. Then
We show that unlike Lie algebras which one-to-one correspond to simply
connected Lie groups, Lie algebroids (integrable or not) one-to-one correspond
to a sort of etale Lie 2-groupoids with 2-connected source fibres. Finally, we
discuss how to lift Lie algebroid morphisms to Lie 2-groupoid morphisms (Lie II
Theorem).
