Here's a very recent thread, from only two weeks ago, telling you how: http://tpt.io/:18718

There are also other threads and some saves that tell you how, which you should be able to find using the search bar.

And here's why it's a bad idea:

They don't work reliably, particularly with particle interactions, and their behaviour may change in the future. The properties of the stacked particles depend on the details of how various things are implemented in the source code, and as such are likely to change (possibly by accident) when the code changes. It is not an intentional feature, and so may not remain unchanged when new features are added or particularly messy parts of the code are rewritten.

For example, interactions between stacked particles depend on which update function the interaction occurs in. If stacked EMP is next to spark, all the EMP particles trigger because EMP checks for spark in its update function. However, if this was moved to the spark update function, then only a single stacked EMP particle would be triggered, not all of them. Why would we move it to the spark update function? Well, since EMP only interacts with spark, moving the interaction to the spark update function would allow the EMP update function to be removed, which should lead to an increase in speed in saves with EMP due to fewer functions being called.

In most cases, there are better ways of making saves do what you want.

@Cocoa_Bits (View Post)

That's supposed to happen. If there are more than 5 particles in the same place, they turn into BHOL. Just as in real life, if you squeeze enough matter too close together, you get a black hole.

(This is to prevent excessive stacking)