Use a plastic straw or stir stick for leveling. Be sure to rinse stir sticks, measuring cups, etc. Also, remind the kids that they should always explain their answers even if they are not explicitly asked for an explanation. To observe the effects of surface tension. Use soap and oil in plastic squeeze bottles to make it easy to add only one drop.
Pepper floats on water.
When you put a drop of oil on the water, the pepper moves aside and the oil flattens out and floats. The oil doesn't flatten beyond a certain point because it's surface tension holds it together. When you add a drop of dish detergent, it also floats on the surface of the water. But it spreads out much thinner than oil and therefore takes up more surface area. Like the oil, it pushes the pepper aside.