Conditional Probability (intuition-related question) I'm having trouble gaining intuition as to why the answer is 2/3 as opposed to 1/2. 
Here is the question:
Alice has five coins in a bag: two coins are normal, two are double-headed, and the last one is double-tailed. She reaches into the bag and randomly pulls out a coin. The coin lands and shows heads face-up. What is the probability that the face-down side is heads? 
So our sample space starts off as {H,T  H,T   H,H   H,H   T,T}
Given that face-up is {H} our probability will be conditioned on the event
{H,T  H,T   H,H   H,H }occurs, which leaves these possibilities -> {H, H, T, T}. 
Intuitively speaking, shouldn't the answer be 2/4? 
The answer uses law of total probability to get the answer, which makes sense mathematically, but I don't get what's wrong with my logic in the intuitive answer. 
Thank you. 
 A: "Intuitive thinking doesn't always give the right answer." is the short answer.

{H,T  H,T   H,H   H,H }occurs, which leaves these possibilities -> {H, H, T, T}

First off I'm not sure what you mean here, but I think something like "Head means we can ignore the double-tailed coin. Then we could have head in two cases and tails in two cases." Your intuitive is wrong simply because in {H,T  H,T   H,H   H,H } the H occurs six times but the T occurs only twice. There is no reduction to {H, H, T, T} I could see.
A: You are of course correct that it can only be one of the four coins as indicated, but when you say that the sample space $HT, HT, HH, HH$ leaves as options $T,T,H,H$ you are effectivly assuming that of the double-headed coins, you  could only have gotten the 'first' head being face-up. But since the 'second' one could be the one that's face-up as well, your possible outcomes for the face-down side are $T,T,H,H,H,H$
A: The sample space is:
$$S=\{HT,HT,H_1H_2,H_2H_1,H_1H_2,H_2H_1\},$$
where the first (second) is top (bottom) and the index numbers are sides.
Hence:
$$P=\frac{n(\text{bottom is head})}{total}=\frac46.$$
A: Our configuration space is
\begin{eqnarray*}
\{H,T \,\,\, H,T \,\,\, H_1,H_2 \,\,\, H_1,H_2 \,\,\, T_1,T_2 \}.
\end{eqnarray*}
There are $6$ possible ways to get a head
\begin{eqnarray*}
\{\color{red}{H},T  \,\,\, \color{red}{H},T  \,\,\, \color{red}{H_1},H_2 \,\,\,  \color{red}{H_1},H_2  \,\,\, \color{red}{H_2},H_1  \,\,\, \color{red}{H_2},H_1 \}.
\end{eqnarray*}
So ... 
