Touch and No-go?

Image by eszter via Flickr

What 2 things, once they've touched, can never return to their normal state?
This is a question that I've been pondering for nearly a month, but I haven't yet come up with an answer that feels right. I did ask my friends what they thought, and their answers are sure good for some laughs:

• Caterpillar & Cocoon (broroth)
• Egg & Sperm (erkinerbyn)
• Flour & Water
• Seed & Soil
• Thin Mint Cookies (fricosuave)
  

I like these because they involve this process of conception... over time, an evolution... or something that dissolves... there is a chemical or biological aspect to this, perhaps.

What say you?