Mathematical Paradoxes: Twin coin flipping

I’ve recently come across some fascinating probability and statistical paradoxes that made me think that our possibly distorted perception of reality and our intuition can lead us to the wrong conclusions quite often.

I will not post any relevant links yet, but I will post some of the paradoxes, some are very common, others are very frustrating, and others very difficult to understand and explain (these are the worst ones!). So here we go, an easy one for warm up, coin flipping:

Flip two coins simultaneously. If one coin is heads, what is the probability that the other coin is also heads?

UPDATE: There is a new post “Twin coin flipping ‘paradox’ revisited” which explains this scenario and includes a javascript that demonstrates the correct results.