We give a new and improved proof that the shrinkage exponent of De Morgan formulae is 2. Namely, we show that for any Boolean function f : {0,1}n → {0,1}, setting each variable out of x1,,xn with probability 1 -- p to a randomly chosen constant, reduces the expected formula size of the function by a factor of O(p2). This result is tight and improves the work of Håstad [SIAM J. C., 1998] by removing logarithmic factors. As a consequence of our results, the function defined by Andreev [MUMB., 1987], A : {0,1}n → {0,1}, which is in P, has formula size at least Ω(n3/log2 n log3 log n). This lower bound is tight (for the function A) up to the log3log n factor, and is the best known lower bound for functions in P. In addition, we strengthen the average-case hardness result of Komargodski et al., we show that the functions defined by Komargodski et al., hr: {0,1}n → {0,1}, which are also in P, cannot be computed correctly on a fraction greater than 1/2 + 2 -- r of the inputs, by De Morgan formulae of size at most n3/r2poly log n, for any parameter r ≤ n{1/3}. The proof relies on a result from quantum query complexity by Laplante et al. [CC, 2006], Hϕyer et al. [STOC, 2007] and Reichardt [SODA, 2011]: for any Boolean function f, Q2(f) &le, O(√L(f)), where Q2(f) is the bounded-error quantum query complexity of f, and L(f) is the minimal size De Morgan formla computing f.