Investing in Bitcoin is a bit risky, considering that real-world uses for the digital token are still limited, and its price can be very volatile. But Bitcoin continues to gain more widespread acceptance among investors, and financial institutions are warming to the cryptocurrency with their launch of spot Bitcoin exchange-traded funds (ETFs) last year.

These ETFs have made it easy for investors to dip their toes into the sometimes wild world of cryptocurrencies. If you're bullish on Bitcoin's long-term prospects, here are two ETFs that are a great place to put $100.

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1. iShares Bitcoin Trust

The iShares Bitcoin Trust (IBIT 1.08%) was one of the first Bitcoin ETFs to launch last year, and it has quickly become one of the most popular among investors. After about a year of existence, the ETF has about $56 billion in assets.

The size of the iShares Bitcoin ETF is notable because it means there's plenty of people buying and selling the ETF shares, allowing for easy liquidity and potentially lower transaction costs. The ETF's backing by BlackRock, a prominent investment company, and Coinbase, a top cryptocurrency exchange for Bitcoin custody, is also noteworthy.

The Bitcoin fund has become popular among some billionaires, too, with Millennium Management's Israel Englander adding 12.6 million shares of the ETF late last year, bringing his total up to 23.5 million shares.

It's also fairly inexpensive to own the iShares Bitcoin Trust. The ETF charges an expense ratio of just 0.25%, which means you'll pay $2.50 annually for every $1,000 invested. That's not the lowest fee available for a Bitcoin ETF, but it's competitive.

2. Grayscale Bitcoin Mini Trust ETF

Another popular option for Bitcoin investors is the Grayscale Bitcoin Mini Trust ETF (BTC 1.05%). If you're wondering why it has "mini" in its name, it's because the ETF was spun off from one of Grayscale's larger Bitcoin ETFs, with 10% of its holdings going to the new, smaller ETF.

One of the most notable things about the Grayscale Bitcoin Mini Trust ETF is that it has one of the lowest fees of Bitcoin ETFs, charging just 0.15%, or $1.50 per $1,000 invested. That's far cheaper than the company's original Grayscale Bitcoin Trust, which has an annual management fee of 1.5%.

Cheaper fees are always important for inventors because it means you'll keep more of your returns as the value of your ETF goes up. For example, if you invested $2,000 into the Grayscale Bitcoin Mini Trust ETF and Bitcoin's value increased 10% over the next year, you'd save about $30 in annual fees, compared to owning the original Grayscale Bitcoin Trust.

Keep this in mind when buying a Bitcoin ETF

Bitcoin's value has soared about 100% over the past year (as of this writing), which has sparked a lot of interest among new crypto investors. But Bitcoin's value can be very volatile. Most recently, the digital token's value dropped nearly 13% between mid-December and early January.

Even if you're investing just $100 into a Bitcoin ETF, make sure you understand that cryptocurrency investments are still speculative. Being able to stomach large price swings up and down is a must, as is keeping a level head when things turn sour.

If you're not comfortable with large price swings but still want a simple way to invest your money, an S&P 500 index fund may be a better option.