Mikayla Huffman

If you take a cup of water and drop an ice cube in it, you'll end up with more water than when you started. But, if you throw a snowball into the cup really hard, you'll splash out a bunch of water. Whether you end up with less or more water than when you started depends on a lot of things- how big your cup is, how much water was in it to start with, how big your ice cube is, how fast you're throwing it into the cup, how much ice is in your ice cube vs how much dust, etc. 

The same thing is true of impacts and atmospheres. Comets are big balls of ice and dust. Asteroids can also have ice in them. When a slow, small comet hits a planet, the ice inside it can turn into gas in the planet's atmosphere (an atmosphere is the shell of gas around the planet). But, if you throw a really big, fast comet at a planet, it can "splash" out gases from the planet's atmosphere. 

Of course, impact alteration is actually more complicated than this metaphor! I've been looking at the models in the literature that try to figure out how different impactors change atmospheres. So far, it looks like they disagree a lot! Yikes!