Take a single pair each of Aces, 2s, 3s, and 4s from a deck of cards for a total of 8 cards. Arrange the cards in a row, such that one card is between the aces, two cards are between the 2s, three cards are between the 3s, and four cards are between the 4s.
Extension 1: Find solutions for 3 pairs (six cards: Aces, 2s and 3s) to 8 pairs (16 cards: Aces, 2s, ..., 8s).
Extension 2: Is there an upper bound? Can you find some N>8, such that N pairs cannot be arranged in this way?
Source: Reader Jimmy Chng Gim Hong, though I've seen it elsewhere.