If all cells have the same genes, explain why skin cells are so different compared to heart cells?
While all cells have the same genes, cells turn on different genes at different times in order to become specialized to their functions.
If you think of DNA like the directions for making proteins, you can see that even though every cell (generally) has the same directions, if cells use different parts of the directions, they will end up being different.
For example, if DNA were like a recipe book, two people could have the same book with all the same recipes, but if they choose different individual recipes to make, they will end up with different foods.
Skin cells are specialized to be quickly shed and replaced, and do not have much mitochondria (which helps produce energy). Muscle cells, conversely, have lots of mitochondria because they need energy to produce movement.
See the images below for more info on skin and muscle cells' shape and general appearance.