In the Pre-Flop Essentials article, there is a chart listing which hands you can open-raise with, and also which hands you can call raises with. Included on the chart are small pairs. When we hold a small pocket pair and are considering calling a raise, in order to go set-mining, there is a simple rule to remember: The Call 20 Rule.
This rule is based on the implied odds of winning a big pot (or even a whole stack) on the rare occasions that we flop a set. "Call 20" means we want implied odds of 20 to 1. That is to say, we will call a bet of X, if we have the chance to win 20X. If the effective stack size is 20 times the price of the call, we can call and go set-mining. If the effective stack is less than 20 times the price of calling, we don't have the implied odds to make a profitable call, so we should fold.

 

Here's an example hand in which the Call 20 Rule is followed.

In the hand above, hero is in late position with pocket deuces, facing a 3bb raise from Villain 8. To see if he can speculate according to the Call 20 Rule, hero first multiplies the raise by 20 to find out the effective stack size required. 20 * 3bb = 60bb. If the actual stacks are larger than required, then a call will be profitable in the long run. Since hero has 80bb and the villain has more than that, hero can call and go set-mining. The big blind also calls, so the flop is seen by three players. Hero didn't flop a set, so he folds to a bet on the flop.

Here's a hand in which hero is lucky enough to flop a set.

Hero only had to call 4c pre-flop. 20 * 4c = 80c, so with a stack of $1.28, hero can make the call according to the Call 20 Rule with his pocket sixes. Many other players also call, so the pot is already quite big on the flop. A villain bets, and hero raises with his set. Unfortunately, everyone folds, so hero doesn't win a whole stack this time, but he still made a nice profit.

Here's a hand in which hero has a small pair, but cannot call pre-flop because the implied odds are not available.

Villain 2 opens UTG for 8c. Although hero has a $1.42 stack, which is almost 20 times the size of the raise, the villain only started the hand with 71c. Since the effective stack is less than 20 times the price of calling, it is best to fold pocket threes here. It's not worth calling 8c, when the maximum that hero can win is only 71c.