I have a table with lots of cells, containing products to buy. Certain cells will open a popup when clicked, which shows similar products. The rest will not (because no similar products exist).
How best can I indicate which cells can be clicked? I'm keen to make it as intuitive as possible. However I should also mention that space is limited in the cells.
Here's a (heavily-edited) screenshot of how it currently works. Users have to click on the cell in the basic grid to see all the similar items. But often they don't realise they can.