A Fibonacci number. There are two blades on a pair of scissors and two sides to a piece of paper.
People have two hands and so do some clocks.
There are two sexes and two sides to an argument. Two-dimensional means that something has just length and width, but no depth. Two's company, three's a crowd all depends on who you happen to be with. Bi- means two. For example, a bicycle has two wheels and a bigamist has two husbands or two wives.