Sharks are predators with extraordinarily acute senses that allow them to detect and track wounded or dying fish. Sharks often select weak, sick, or injured prey because they are easier to catch than healthy prey. A shark’s nostrils, or olfactory organs, help a shark smell their prey (SF Fig. 2.15.1).

The concentration of an odor in water is measured in parts per million (one odor molecule for every million molecules of H₂O). Sharks can smell blood from hundreds of meters away—in concentrations as low as one part per million (ppm).

One part per million (ppm) is the same as

• one inch in 16 miles,
• one minute in two years,
• one pinch of salt in 2,000 pounds of potato chips, and
• one cent in 10,000 dollars.

Compare your sense of smell to that of a shark.

Materials

• Eight identical clear cups
• Permanent marker
• Tap water
• Large measuring cup, marked in mL
• 5 or 10 mL measuring spoon
• Spoon or stir stick
• Tomato or lemon juice

Procedure

Safety Note: Use food-safe cups and spoons that have not been used with laboratory chemical or biological substances.

1. Label the cups one through seven.
    
2. Use the measuring cup to carefully measure 90 mL of water into the cups numbered two through eight.
    
3. Use the measuring cup to measure 60 mL of tomato or lemon juice into cup number one.
   
   1. The tomato juice represents blood from a wounded fish.
   2. The lemon juice represents bodily fluids that a shark might smell in the ocean.
       
4. Use the measuring spoon to transfer 10 mL of the juice from cup number one into cup number two. Mix cup number two well.
   
   1. Repeat this step until you have put 10 mL from cup number six into cup number seven.
   2. Cup number eight is your control; this cup only has tap water in it.
       
5. Starting with cup number eight, smell each of the cups to see when you are able to detect the juice in the cups.
    
6. (Optional) Use your other senses to make observations about the liquids in the cups as instructed by your teacher. Two examples of observations are listed below.
   
   1. Look at the appearance, or color, of the solutions by drawing circles and coloring them with a crayon to correspond to the color in each cup. You can look at paint swatches to compare colors. To observe the color of the liquid in the cup, you may have to put a sheet of white paper under the cup.
   2. Take a small sip of the liquids from each cup, starting from cup eight.