To Compare the level of water / liquid, you can use CUP..cup is the easiest and cheapest that you can get..it is advisable to use transparent cup as it would be more easier for you to see the level of the water from outside the cup.Below is some example of the cup that you can use..
next you need some water which you can get it from any water tap..and fill in the cups with different level of water to show some differences..it is highly suggested that you fill in the cups with the following :
2. half full
After that show to the class and let them guess the answer..
after this, you can ask the kids to do some exercise which can get from the link below