Secret:

Have the volunteer pick a number.
Write it onto the sugar cube with a pencil (press hard).
Then, hold the cube between your thumb and one finger.
Hold it so the number transfers onto your thumb and say," Now I will put this cube into the cup".
Press the cube as hard as possible so the number is on your thumb.
Put the cube into the water and hold the volunteer's hand above the water, make sure your thumb is in their palm so the number from your finger transfers onto the volunteer's hand. This is a good trick to show smaller kids because they are less likely to figure out the trick.