Bring 1 ½ cups of water to a boil. Add 15 drops of food colouring (or a different amount of your choice) and stir until combined.
Place unflavoured gelatin powder in a mixing bowl. Pour boiled water onto gelatin and quickly whisk them together, continuing until the powder is completely dissolved.
Add ¾ cup of cold water and whisk until combined.
Line a baking dish with parchment paper, then slowly pour the gelatin mixture into the dish. Allow mixture to set for 3-4 hours in the fridge.
Once the gelatin has set, take the baking dish out of the fridge and use a butter knife to cut it into cubes. Then, give the cubes to your child to play with!
Notes
To make the blocks more solid, reduce the amount of water. To make them softer (for younger babies and toddlers), increase the amount of water. Unflavoured gelatin powder can be replaced with two 4-ounce boxes of flavoured Jell-O powder.