Hmm, right. Bolzanos Theorem, a useless mathematical theorem:
For two real a and b , a < b , and a function f that is continuous on a closed interval [a , b ] such that f(a) and f(b) are of opposite signs, there exists a number x0 belonging to the interval [a , b ] with f(x0) =0.
This basically means that if you have a continuous function (the graph of the function isn't "broken" anywhere) in 2 dimensions with one point below the x-axis and one point above the x-axis, then that function will cross the x-axis somewhere. Hard to imagine huh?