Fresh water stingrays live in the rivers of the Amazon Rainforest. Rivers are freshwater bodies of water, meaning that they have a low concentration of salt or no salt at all. In this experiment you will see how salt water is denser then fresh water. Density is a measure of the mass per unit volume of a substance.

• Fill a tall drinking glass of tap water until it is about half full.
• Place an egg in the glass of water. It should sink.
• Take out the egg and add 1 tablespoon of salt, stirring until the salt dissolves.
• Place your egg back in and see if it floats. If it doesn’t, add another tablespoon of salt to the water until it does.
• Record how many tablespoons of salt you needed for your egg to float.
• Next, carefully pour more tap water until the glass is nearly full. Try not to disturb the salt water with the tap water. See if you can get the egg to float between the fresh and the salt water.

The egg is sinking in the tap (fresh) water because its density is greater then the density of the fresh water (it has more molecules per square centimetre). When you started to dissolve the salt in the water, you were increasing the density of the water. When the salt water becomes denser, the egg will float.

Now try this experiment using other objects; e.g. a carrot slice, a paper clip. Remember to add one tablespoon of the salt at at time and mix well until the salt has dissolved. Add your object to see if it floats or sinks. Keep adding a tablespoon of salt at a time until you can get your object to float. Create a data table to track how much salt you needed to add for each object to float.

Freshwater stingrays are a relative of sharks and have cartilaginous skeletons rather than bones. As their name suggests, they have a sharp venomous barb on the base of their tails that can easily penetrate human skin and bone. They are non aggressive but will sting you if you step on them or harass them.

Draw two pictures of a sting ray; one from the top view and the other from the bottom view. Show where these parts are located:

• Eyes
• Snout
• Gills
• Tail
• Stinger
• Fins
• Spiracle

Next paint your sting ray. Don’t worry about it being realistic. Have fun and use whatever colours you choose. You can even paint crazy patterns on your sting ray.

There are many important rivers in the world. Here is a list of ten of them. For each river research and write a short paragraph about it. Include where it is located, where it starts and ends, interesting facts, what types of plants and mammals live on that river.

• Sepik
• Mississippi
• Volga
• Zambezi
• Mekong
• Ganges
• Danube
• Yangtze
• Nile
• Amazon

Sadly, stingray populations around the world are in decline.

Write a research paper which includes where stingrays can be found around the world, why they are in decline and ideas/solutions to combat their decreasing numbers.