Put the egg white in a large bowl and whisk until foamy. Stir in the lemon juice.
Gradually stir in the icing sugar with a wooden spoon until really stiff, like dough. Or, dust the work surface with some icing sugar and knead the sugar into the mixture until you get the right consistency.
Divide the mixture in half. Add a dot of pink food colouring to one half, then knead in to evenly distribute the colour. Add a tiny bit more colouring if you want a stronger colour. Leave the second half plain.
Break off a walnut-sized piece of the mixture and roll into a rounded cone shape, then flatten one side so that the mouse can sit on a flat surface without toppling over. Pinch out little ears on top of the narrow end, then squeeze the same end into a nose. Press chocolate sprinkles into the face below the ears to make eyes. Cut a length of kitchen string about 4cm long, and push it into the round end of the mouse to make a tail.
Use a cocktail stick to dab a tiny amount of pink food colouring onto the nose, then put the mouse on a baking sheet lined with baking parchment.
Repeat the process with the remaining pink and white mixtures, then leave the mice to dry out for at least 12 hrs.