A simple equal-arm lever
Make a wooden base 15 cm square and 2 cm thick. In the centre of the base fasten one more block of wood 4 cm square and 3 cm thick. Fasten to two sides of these blocks two uprights 15 cm long, 3.5 cm wide and 1 cm thick. These may be fastened to the small block by means of screws. In the top end of each up- right cut a narrow fit with a thin blade saw. The slot should be a little less than 2 cm deep or just deep sufficient to hold a used razor blade with 2 or 3 mm extending above the top of the upright.