1. Make a model of bicyclo[1.1.0]butane. I suggest that you use flexible (and preferably inexpensive!) models that can be bent easily to accommodate formation of highly strained rings.
Be certain to construct your model of this molecule with a cis ring junction. Otherwise, be prepared to sweep up the pieces from the floor.
2. Make another model of bicyclo[1.1.0]butane that is the mirror image of the first molecular model that you constructed in response to #1, above.
3. Are the two models that you constructed mutually superimposable? If so, then bicyclo[1.1.0'butane is achiral. If not, then this molecule is chiral.