IMO, the best way to explain floating point is to play with a tiny float. With an 8-bit float (1 bit sign, 4 bits exponent, 3 bits mantissa, exponent bias 7), there are only 256 possible values. One can write by hand a table with the corresponding value for each of the 256 possibilities, and get a feel to how it really works.

(I got the 1+4+3 from http://www.toves.org/books/float/, I don't know if it's the best allocation for the bits; but for didactic purposes, it works.)