The maximum deflection possible for a simply supported object is the following equation:

\Delta_{max}=\frac{PL^3}{48AE}

I decided to choose the small wooden work table I have in my room, with it’s dimensions being 31½” × 15¾” × ⅝”. We’ll need to know the moment of inertia, here denoted by “I”, in regards to the dimensions perpendicular to the bending, a.k.a the bending stiffness of the cross section of the wooden table face.

I=\frac{bh}{12}

Solving these two:

$I=\frac{b{h}^3}{12}=\frac{\mathrm{(15¾”)}{\mathrm{(⅝”)}}^3}{12}$ = 0.3204 in⁴

I shall use P to be equal to a couple ounces of force (⅛ pound-force).

${\mathrm{\Delta }}_{\max }=\frac{P{L}^3}{48EI}=\frac{\mathrm{(2\; ozf)}{\mathrm{(31½”)}}^3}{48\mathrm{(1½\; million\; psi)}\mathrm{(0.3204\; in⁴)}}$ = 0.000173 in

or 173 millionths of an inch just from a simple finger tap, a deflection value of which is simply invisible to the human eye.