Mels are found in tower formations and move from side to side, turning the other way when they encounter a wall or another stack of Mels. They will damage Yoshi if they bump into him. However, Yoshi is able to stand on the topmost Mel and move along with the entire stack. He is also able to eat Mels one by one and turn them into eggs, defeating them in the process.
There is another variant of Mels which are wrapped in tinfoil. These are resistant to most of Yoshi's attacks and cannot be eaten. However, an entire stack of Mels can be collapsed by running into it on Poochy.
Most Mels in a stack show the same expression on their faces at one given time, which changes depending on the situation. Uncovered Mels normally display a happy expression, while tinfoiled Mels look angry. Both variants turn to a surprised or worried expression while Yoshi stands on them.