The Globe and Mail reports in its Thursday edition that the United States and Canada have seen rapid growth of exchange-traded funds built around single stocks, such as Tesla. The Globe's Andrew Galbraith writes that while these funds in the U.S. tend to involve complex strategies and higher leverage that magnify both potential gains and losses, Canadian single-stock ETFs have focused on covered calls, in part because of tax advantages. Ninety-eight per cent of Canadian single-stock ETFs employ covered calls, according to TD Securities. Covered-call ETF managers sell call options on underlying assets, giving buyers of those options the right to buy the assets by a specific date at an agreed upon price, called the exercise price. If the asset price is below the exercise price on the option's expiration date, the option expires worthless and the ETF managers retain the asset and the income, called the premium, from the option sale. When an asset's price falls, the option income can soften the blow. If the asset price rises above the strike price, a covered-call strategy can cap an investor's gains. However, they still receive the premium from selling the options. Sales of covered-call ETFs have grown to $57-billion today.