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A bottle says five percent solution. Five percent of what? The little letters beside the number tell us. For mass by mass, count grams of solute in a hundred grams of solution. Ten grams of salt plus ninety grams of water gives ten percent. For mass by volume, count grams of solute in a hundred millilitres of solution. Notice: now the denominator uses volume. Five grams of glucose, with enough water to make a hundred millilitres of solution, gives five percent mass by volume. For volume by volume, count millilitres of solute in a hundred millilitres of solution. Five millilitres in that final volume gives five percent. Each calculation is solute divided by solution, times a hundred. Check the units and count the whole solution—not just the water.

Can one paper filter catch everything in water? Watch what happens when we filter a mixture containing sand and dissolved salt. The sand has not dissolved. Its grains are suspended in the water, and many can settle if we leave it alone. Pour the mixture onto filter paper. The paper's small openings stop the larger grains. This collected material is the residue. Water passes through into the container below. The liquid that passes through is called the filtrate. Look closer: dissolved salt travels with the water. Some very fine suspended particles can also pass an ordinary filter, leaving the water cloudy. So filtration works when particles are large enough to be trapped. Clear-looking water may still contain dissolved substances or germs. Filtering alone does not make it safe to drink.

Shine a torch at a mirror. Where will the light go next? Let's draw its path. Light travels towards the mirror in a straight line. This is the incoming, or incident, ray. When it hits the mirror, it changes direction and travels away. That's reflection. But how do we find the new direction? First, draw a line at a right angle to the mirror, exactly where the light hits. We call this line the normal. Now measure the angle between the incoming ray and the normal. The outgoing ray makes the same angle on the other side. Here, forty-five degrees in means forty-five degrees out. Both rays and the normal lie in the same plane, like our drawing. So remember: measure from the normal, not from the mirror. Equal angles tell us where the reflected light will go.