Different diets to compare

Suppose you have 4 different diets which you want to compare. The diets are labeled Diet A, Diet B, Diet C, and Diet D. You are interested in how the diets affect cognitive function of mice. The cognitive function is the time in seconds that it takes a mouse to run a maze. You have 20 mice available for the experiment. You will use 5 on each diet. The 20 mice are in a large cage until you are ready to begin the experiment, at which time they will be transferred to cages. How should you use randomization to assign the mice to the four treatment groups in a completely randomized design? Describe in a paragraph.