To prove to yourself what temperature water freezes and boils at, fill a pan with ice from the freezer. Stick a thermometer in it (not a fever thermometer! Use an outside thermometer for the freezing part and a candy thermometer that goes up as high as boiling water for the boiling part) and you'll see that the ice is about zero Celsius or 32 degrees Fahrenheit. Now put it on the stove and turn the burner on (with adult permission). Don't leave the thermometer in it or it will break. But every minute or so, take the pot off the stove and see how hot the water is. How hot is it when the water is boiling?
You can also use thermometers to keep track of temperatures over time. What's the hottest day of the year where you are? What's the coldest day? What is the difference between the high temperature in the daytime and the low temperature at night?