Put 2 Tbsp water to a bowl. Add food coloring if you'd like. Add 5 Tbsp cornstarch to the bowl and mix with your fingers until smooth.
Magic ooze is a non-Newtonian fluid. This means that the more pressure you put on it, the thicker it becomes. Scientists aren't sure why non-Newtonian fluids do what they do.
WARNING: Dispose of this stuff in the garbage. Do NOT put it down the sink. It will clog the drain.