Did you have cake and ice cream for your birthday?  I bet you did.  Can you imagine a time before ice cream?  Yes, there was such a time.  Ice cream is a recent invention.  It was invented only a few hundred years ago, even before refrigerators and freezers were invented.

How is it possible to make ice cream without a freezer?  The key is salt.  Ice cream became possible because some clever people saw that salt melts ice.

How do salt, water and ice cause milk to turn into ice cream?  You will understand by doing it.

Get a bucket of ice.  Let the ice melt.  Dip your finger in it -- that's 0 degrees Celsius (32 degrees Fahrenheit) -- the temperature that ice melts at.  The ice doesn't get hotter; it just changes to liquid water.

Get another bucket of ice.  Add lots of salt to it.  You'll see that this ice melts faster.  Dip your finger in it -- ouch!  The temperature is much colder.  Ice sucks up heat from the salty water and the temperature of the salty water drops -- below freezing.  Ice keeps melting but the water stops freezing.

Why doesn't this water freeze?  Because it has salt in it.  Salt makes it harder for the water to lock into an ice crystal.

Now, place a smaller container with milk in the bucket that has ice, water and salt.  Heat flows out of the milk and melts more ice.  The milk gets very cold, colder than freezing.  It turns into ice cream.  And the best ice cream comes from stirring the milk while it is freezing.  That way, the ice crystals that form are small, making it smooth and creamy.

We don't know who discovered that salt melts ice.  But thanks to these observant people, we can all enjoy ice cream.

Click to Make Your Own Ice Cream.