"Assuming that you haven't an instrument sensitive enough to measure the thickness of a sample of foil directly, then you will have to measure something easier to measure and then do some maths. Try any of the following:
1. Measure the length and width of a piece of aluminium foil. Look up the density of aluminium in Kaye and Laby (or, if you can find a suitable chunk of aluminium, a pair of scales and a graduated cylinder, measure it yourself; then look it up anyway to see how accurate your measurement was). Weigh your piece of foil (feel free to fold it up or crumple it into a ball to make it fit on the scales; as long as you don't tear any off, it will still weigh the same) and calculate its volume using mass / density. Divide this by the area you measured earlier to get the thickness.
2. Place several layers of foil in intimate contact with one another and measure the thickness of the stack. Divide by the number of layers to get the thickness of each layer. Or, keep adding layers and measuring; plot a graph of thickness vs. number of layers and measure the slope.
3. Use a lens to project a magnified image of the edge of a sample of foil onto a screen. Divide the thickness of the foil *in the image* by the magnification factor (remember, light travels in straight lines; so, by the properties of similar triangles, the magnification factor will be equal to the ratio of the distances of image and object from the lens) to get the thickness of the *real* foil.
Actually, if this is a coursework project, do all three (plus any more that you can think of or that other people suggest) and compare the results of each method. Where might errors creep in? What can you do to minimise them?"