We have little medicine cups of the type used in hospitals. We will fill some of them with water, some with soapy water, some with vinegar, and some with cooking oil. They need to be filled until the liquid just reaches the top edge.

Make a list of the liquids, so we can write information about each one. The fun part is to add to each liquid, just one drop (with medicine droppers) at a time, counting the drops. When a cup runs over, stop adding to it, and write down the number of drops added before it overflowed.

Did each liquid take the same amount of added drops? Did they all rise a bit above the rim of the cup? Did some rise more than others? What do you think it is that keeps them from overflowing immediately?

Which liquid seems to have the strongest surface tension?